Method of modifying the properties of a metal matrix composite body

ABSTRACT

The present invention relates to modifying the properties of a metal matrix composite body by a post formation process treatment and/or a substantially contiguous modification treatment. The post formation process treatment may be applicable to a variety of metal matrix composite bodies produced by various techniques, and is particularly applicable to modifying the properties of a metal matrix composite body produced by a spontaneous infiltration technique. The substantially contiguous modification process may also be used primarily in conjunction with metal matrix composite bodies produced according to a spontaneous infiltration technique. Particularly, at least a portion of the matrix metal of the metal matrix composite body and/or the filler material of the metal matrix composite body is modified or altered during and/or after the formation process.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation-in-part application of InternationalPatent Application No. PCT/US93/06065, filed on Jun. 25, 1993, whichdesignated the U.S., and which is a continuation-in-part of U.S. patentapplication Ser. No. 07/904,757, filed Jun. 26, 1992 and now abandoned,which is a continuation-in-part application of U.S. patent applicationSer. No. 07/841,241, filed Feb. 24, 1992 and now issued as U.S. Pat. No.5,301,738, which is a continuation of U.S. patent application Ser. No.07/520,944, filed May 9, 1990 and now abandoned, which is acontinuation-in-part of U.S. patent application Ser. No. 07/269,309,filed Nov. 10, 1988, which issued on Mar. 19, 1991, as U.S. Pat. No.5,000,248, in the names of Marc S. Newkirk et al., and all of which areentitled "A Method of Modifying the Properties of a Metal MatrixComposite Body".

FIELD OF THE INVENTION

The present invention relates to modifying the properties of a metalmatrix composite body by a post formation process treatment and/or asubstantially contiguous modification treatment. The post formationprocess treatment may be applicable to a variety of metal matrixcomposite bodies produced by various techniques, and is particularlyapplicable to modifying the properties of a metal matrix composite bodyproduced by a spontaneous infiltration technique. The substantiallycontiguous modification process may also be used with a variety oftechniques and is particularly applicable in conjunction with metalmatrix composite bodies produced according to a spontaneous infiltrationtechnique. Particularly, at least a portion of the matrix metal of themetal matrix composite body and/or the filler material of the metalmatrix composite body is modified or altered during and/or after theformation process.

BACKGROUND OF THE INVENTION

Composite products comprising a metal matrix and a strengthening orreinforcing phase such as ceramic particulates, whiskers, fibers or thelike, show great promise for a variety of applications because theycombine some of the stiffness and wear resistance of the reinforcingphase with the ductility and toughness of the metal matrix. Generally, ametal matrix composite will show an improvement in such properties asstrength, stiffness, contact wear resistance, and elevated temperaturestrength retention relative to the matrix metal in monolithic form, butthe degree to which any given property may be improved depends largelyon the specific constituents, their volume or weight fraction, and howthey are processed in forming the composite. In some instances, thecomposite also may be lighter in weight than the matrix metal per se.Aluminum matrix composites reinforced with ceramics such as siliconcarbide in particulate, platelet, or whisker form, for example, are ofinterest because of their higher stiffness, wear resistance and hightemperature strength relative to aluminum.

Various metallurgical processes have been described for the fabricationof aluminum matrix composites, including methods based on powdermetallurgy techniques and liquid-metal infiltration techniques whichmake use of pressure casting, vacuum casting, stirring, and wettingagents. With powder metallurgy techniques, the metal in the form of apowder and the reinforcing material in the form of a powder, whiskers,chopped fibers, etc., are admixed and then either cold-pressed andsintered, or hot-pressed. The maximum ceramic volume fraction in siliconcarbide reinforced aluminum matrix composites produced by this methodhas been reported to be about 25 volume percent in the case of whiskers,and about 40 volume percent in the case of particulates.

The production of metal matrix composites by powder metallurgytechniques utilizing conventional processes imposes certain limitationswith respect to the characteristics of the products attainable. Thevolume fraction of the ceramic phase in the composite is limitedtypically, in the case of particulates, to about 40 percent. Also, thepressing operation poses a limit on the practical size attainable. Onlyrelatively simple product shapes are possible without subsequentprocessing (e.g., forming or machining) or without resorting to complexpresses. Also, nonuniform shrinkage during sintering can occur, as wellas nonuniformity of microstructure due to segregation in the compactsand grain growth.

J. C. Cannell et al. U.S. Pat. No. 3,970,136, granted Jul. 20, 1976,describes a process for forming a metal matrix composite incorporating afibrous reinforcement, e.g. silicon carbide or alumina whiskers, havinga predetermined pattern of fiber orientation. The composite is made byplacing parallel mats or felts of coplanar fibers in a mold with areservoir of molten matrix metal, e.g., aluminum, between at least someof the mats, and applying pressure to force molten metal to penetratethe mats and surround the oriented fibers. Molten metal may be pouredonto the stack of mats while being forced under pressure to flow betweenthe mats. Loadings of up to about 50% by volume of reinforcing fibers inthe composite have been reported.

The above-described infiltration process, in view of its dependence onoutside pressure to force the molten matrix metal through the stack offibrous mats, is subject to the vagaries of pressure-induced flowprocesses, i.e., possible non-uniformity of matrix formation, porosity,etc. Non-uniformity of properties is possible even though molten metalmay be introduced at a multiplicity of sites within the fibrous array.Consequently, complicated mat/reservoir arrays and flow pathways need tobe provided to achieve adequate and uniform penetration of the stack offiber mats. Also, the aforesaid pressure-infiltration method allows foronly a relatively low reinforcement to matrix volume fraction to beachieved because of difficulty of infiltrating a large mat volume. Stillfurther, molds are required to contain the molten metal under pressure,which adds to the expense of the process. Finally, the aforesaidprocess, limited to infiltrating aligned particles or fibers, is notdirected to formation of aluminum metal matrix composites reinforcedwith materials in the form of randomly oriented particles, whiskers orfibers.

In the fabrication of aluminum matrix-alumina filled composites,aluminum does not readily wet alumina, thereby making it difficult toform a coherent product. Various solutions to this problem have beensuggested. One such approach is to coat the alumina with a metal (e.g.,nickel or tungsten), which is then hot-pressed along with the aluminum.In another technique, the aluminum is alloyed with lithium, and thealumina may be coated with silica. However, these composites exhibitvariations in properties, or the coatings can degrade the filler, or thematrix contains lithium which can affect the matrix properties.

R. W. Grimshaw et al. U.S. Pat. No. 4,232,091, overcomes certaindifficulties in the art which are encountered in the production ofaluminum matrix-alumina composites. This patent describes applyingpressures of 75-375 kg/cm² to force molten aluminum (or molten aluminumalloy) into a fibrous or whisker mat of alumina which has been preheatedto 700° to 1050° C. The maximum volume ratio of alumina to metal in theresulting solid casting was 0.25/1. Because of its dependency on outsideforce to accomplish infiltration, this process is subject to many of thesame deficiencies as that of Cannell et al.

European Patent Application Publication No. 115,742 describes makingaluminum-alumina composites, especially useful as electrolytic cellcomponents, by filling the voids of a preformed alumina matrix withmolten aluminum. The application emphasizes the non-wettability ofalumina by aluminum, and therefore various techniques are employed towet the alumina throughout the preform. For example, the alumina iscoated with a wetting agent of a diboride of titanium, zirconium,hafnium, or niobium, or with a metal, i.e., lithium, magnesium, calcium,titanium, chromium, iron, cobalt, nickel, zirconium, or hafnium. Inertatmospheres, such as argon, are employed to facilitate wetting. Thisreference also shows applying pressure to cause molten aluminum topenetrate an uncoated matrix. In this aspect, infiltration isaccomplished by evacuating the pores and then applying pressure to themolten aluminum in an inert atmosphere, e.g., argon. Alternatively, thepreform can be infiltrated by vapor-phase aluminum deposition to wet thesurface prior to filling the voids by infiltration with molten aluminum.To assure retention of the aluminum in the pores of the preform, heattreatment, e.g., at 1400° to 1800° C., in either a vacuum or in argon isrequired. Otherwise, either exposure of the pressure infiltratedmaterial to gas or removal of the infiltration pressure will cause lossof aluminum from the body.

The use of wetting agents to effect infiltration of an alumina componentin an electrolytic cell with molten metal is also shown in EuropeanPatent Application Publication No. 94353. This publication describesproduction of aluminum by electrowinning with a cell having a cathodiccurrent feeder as a cell liner or substrate. In order to protect thissubstrate from molten cryolite, a thin coating of a mixture of a wettingagent and solubility suppressor is applied to the alumina substrateprior to start-up of the cell or while immersed in the molten aluminumproduced by the electrolytic process. Wetting agents disclosed aretitanium, zirconium, hafnium, silicon, magnesium, vanadium, chromium,niobium, or calcium, and titanium is stated as the preferred agent.Compounds of boron, carbon and nitrogen are described as being useful insuppressing the solubility of the wetting agents in molten aluminum. Thereference, however, does not suggest the production of metal matrixcomposites, nor does it suggest the formation of such a composite in,for example, a nitrogen atmosphere.

In addition to application of pressure and wetting agents, it has beendisclosed that an applied vacuum will aid the penetration of moltenaluminum into a porous ceramic compact. For example, R. L. LandinghamU.S. Pat. No. 3,718,441, granted Feb. 27, 1973, reports infiltration ofa ceramic compact (e.g., boron carbide, alumina and beryllia) witheither molten aluminum, beryllium, magnesium, titanium, vanadium, nickelor chromium under a vacuum of less than 10⁻⁶ torr. A vacuum of 10⁻² to10⁻⁶ tort resulted in poor wetting of the ceramic by the molten metal tothe extent that the metal did not flow freely into the ceramic voidspaces. However, wetting was said to have improved when the vacuum wasreduced to less than 10⁻⁶ torr.

G. E. Gazza et al. U.S. Pat. No. 3,864,154, granted Feb. 4, 1975, alsoshows the use of vacuum to achieve infiltration. This patent describesloading a cold-pressed compact of AlB₁₂ powder onto a bed ofcold-pressed aluminum powder. Additional aluminum was then positioned ontop of the AlB₁₂ powder compact. The crucible, loaded with the AlB₁₂compact "sandwiched" between the layers of aluminum powder, was placedin a vacuum furnace. The furnace was evacuated to approximately 10⁻⁵torr to permit outgassing. The temperature was subsequently raised to1100° C. and maintained for a period of 3 hours. At these conditions,the molten aluminum penetrated the porous AlB₁₂ compact.

John N. Reding et al. U.S. Pat. No. 3,364,976, granted Jan. 23, 1968,discloses the concept of creating a self-generated vacuum in a body toenhance penetration of a molten metal into the body. Specifically, it isdisclosed that a body, e.g., a graphite mold, a steel mold, or a porousrefractory material, is entirely submerged in a molten metal. In thecase of a mold, the mold cavity, which is filled with a gas reactivewith the metal, communicates with the externally located molten metalthrough at least one orifice in the mold. When the mold is immersed intothe melt, filling of the cavity occurs as the self-generated vacuum isproduced from the reaction between the gas in the cavity and the moltenmetal. Particularly, the vacuum is a result of the formation of a solidoxidized form of the metal. Thus, Reding et al. disclose that it isessential to induce a reaction between gas in the cavity and the moltenmetal. However, utilizing a mold to create a vacuum may be undesirablebecause of the inherent limitations associated with use of a mold. Moldsmust first be machined into a particular shape; then finished, machinedto produce an acceptable casting surface on the mold; then assembledprior to their use; then disassembled after their use to remove the castpiece therefrom; and thereafter reclaim the mold, which most likelywould include refinishing surfaces of the mold or discarding the mold ifit is no longer acceptable for use. Machining of a mold into a complexshape can be very costly and time-consuming. Moreover, removal of aformed piece from a complex-shaped mold can also be difficult (i.e.,cast pieces having a complex shape could be broken when removed from themold). Still further, while there is a suggestion that a porousrefractory material can be immersed directly in a molten metal withoutthe need for a mold, the refractory material would have to be anintegral piece because there is no provision for infiltrating a loose orseparated porous material absent the use of a container mold (i.e., itis generally believed that the particulate material would typicallydisassociate or float apart when placed in a molten metal). Stillfurther, if it was desired to infiltrate a particulate material orloosely formed preform precautions should be taken so that theinfiltrating metal does not displace at least portions of theparticulate or preform resulting in a non-homogeneous microstructure.

Accordingly, there has been a long felt need for a simple and reliableprocess to produce shaped metal matrix composites which does not relyupon the use of applied pressure or vacuum (whether externally appliedor internally created), or damaging wetting agents to create a metalmatrix embedding another material such as a ceramic material. Moreover,there has been a long felt need to minimize the amount of finalmachining operations needed to produce a metal matrix composite body.The present invention satisfies these needs by providing a spontaneousinfiltration mechanism for infiltrating a material (e.g., a ceramicmaterial), which is formed into a preform, with molten matrix metal(e.g., aluminum) in the presence of an infiltrating atmosphere (e.g.,nitrogen) under normal atmospheric pressures, so long as an infiltrationenhancer is present at least at some point during the process. Inaddition, the present invention permits at least one property of a metalmatrix composite body to be modified in a desirable manner.

Description of Commonly Owned U.S. Patents and Patent Applications

The subject matter of this application is related to that of severalco-owned Patents and several other copending and co-owned patentapplications. Particularly, the patents and other copending patentapplications describe novel methods for making metal matrix compositematerials (hereinafter sometimes referred to as "Commonly Owned MetalMatrix Patents and Patent Applications").

A novel method of making a metal matrix composite material is disclosedin Commonly Owned U.S. patent application Ser. No. 07/049,171, filed May13, 1987, in the names of White et al., and entitled "Metal MatrixComposites", now U.S. Pat. No. 4,828,008, which issued on May 9, 1989,and which published in the EPO on Nov. 17, 1988, as Publication No. 0291 441. According to the method of the White et al. invention, a metalmatrix composite is produced by infiltrating a permeable mass of fillermaterial (e.g., a ceramic or a ceramic-coated material) with moltenaluminum containing at least about 1 percent by weight magnesium, andpreferably at least about 3 percent by weight magnesium. Infiltrationoccurs spontaneously without the application of external pressure orvacuum. A supply of the molten metal alloy is contacted with the mass offiller material at a temperature of at least about 675° C. in thepresence of a gas comprising from about 10 to 100 percent, andpreferably at least about 50 percent, nitrogen by volume, and aremainder of the gas, if any, being a nonoxidizing gas, e.g., argon.Under these conditions, the molten aluminum alloy infiltrates theceramic mass under normal atmospheric pressures to form an aluminum (oraluminum alloy) matrix composite. When the desired amount of fillermaterial has been infiltrated with the molten aluminum alloy, thetemperature is lowered to solidify the alloy, thereby forming a solidmetal matrix structure that embeds the reinforcing filler material.Usually, and preferably, the supply of molten alloy delivered will besufficient to permit the infiltration to proceed essentially to theboundaries of the mass of filler material. The amount of filler materialin the aluminum matrix composites produced according to the White et al.invention may be exceedingly high. In this respect, filler to alloyvolumetric ratios of greater than 1:1 may be achieved.

Under the process conditions in the aforesaid White et al. invention,aluminum nitride can form as a discontinuous phase dispersed throughoutthe aluminum matrix. The amount of nitride in the aluminum matrix mayvary depending on such factors as temperature, alloy composition, gascomposition and filler material. Thus, by controlling one or more suchfactors in the system, it is possible to tailor certain properties ofthe composite. For some end use applications, however, it may bedesirable that the composite contain little or substantially no aluminumnitride.

It has been observed that higher temperatures favor infiltration butrender the process more conducive to nitride formation. The White et al.invention allows the choice of a balance between infiltration kineticsand nitride formation.

An example of suitable barrier means for use with metal matrix compositeformation is described in Commonly Owned U.S. patent application Ser.No. 07/141,642, filed Jan. 7, 1988, in the names of Michael K.Aghajanian et al., and entitled "Method of Making Metal Matrix CompositeWith the Use of a Barrier", now U.S. Pat. No. 4,935,055, which issued onJun. 19, 1990, and which published in the EPO on Jul. 12, 1989, asPublication No. 0 323 945. According to the method of this Aghajanian etal. invention, a barrier means (e.g., particulate titanium diboride or agraphite material such as a flexible graphite tape product sold by UnionCarbide under the trade name Grafoil®) is disposed on a defined surfaceboundary of a filler material and matrix alloy infiltrates up to theboundary defined by the barrier means. The barrier means is used toinhibit, prevent, or terminate infiltration of the molten alloy, therebyproviding net, or near net, shapes in the resultant metal matrixcomposite. Accordingly, the formed metal matrix composite bodies have anouter shape which substantially corresponds to the inner shape of thebarrier means.

The method of U.S. Pat. No. 4,828,008 was improved upon by CommonlyOwned and Copending U.S. patent application Ser. No. 07/994,064, filedDec. 18, 1992, which is a continuation of U.S. patent application Ser.No. 07/759,745, filed Sep. 12, 1991, now abandoned, which is acontinuation of U.S. patent application Ser. No. 07/517,541, filed Apr.24, 1990, now abandoned, which is a continuation of U.S. patentapplication Ser. No. 07/168,284, filed Mar. 15, 1988, now abandoned, andwhich published in the EPO on Sep. 20, 1989, as Publication No. 0333629,all in the names of Michael K. Aghajanian and Marc S. Newkirk and all ofwhich are entitled "Metal Matrix Composites and Techniques for Makingthe Same". In accordance with the methods disclosed in this U.S. patentapplication, a matrix metal alloy is present as a first source of metaland as a reservoir of matrix metal alloy which communicates with thefirst source of molten metal due to, for example, gravity flow.Particularly, under the conditions described in this patent application,the first source of molten matrix alloy begins to infiltrate the mass offiller material under normal atmospheric pressures and thus begins theformation of a metal matrix composite. The first source of molten matrixmetal alloy is consumed during its infiltration into the mass of fillermaterial and, if desired, can be replenished, preferably by a continuousmeans, from the reservoir of molten matrix metal as the spontaneousinfiltration continues. When a desired amount of permeable filler hasbeen spontaneously infiltrated by the molten matrix alloy, thetemperature is lowered to solidify the alloy, thereby forming a solidmetal matrix structure that embeds the reinforcing filler material. Itshould be understood that the use of a reservoir of metal is simply oneembodiment of the invention described in this patent application and itis not necessary to combine the reservoir embodiment with each of thealternate embodiments of the invention disclosed therein, some of whichcould also be beneficial to use in combination with the presentinvention.

The reservoir of metal can be present in an amount such that it providesfor a sufficient amount of metal to infiltrate the permeable mass offiller material to a predetermined extent. Alternatively, an optionalbarrier means can contact the permeable mass of filler on at least oneside thereof to define a surface boundary.

Moreover, while the supply of molten matrix alloy delivered should be atleast sufficient to permit spontaneous infiltration to proceedessentially to the boundaries (e.g., barriers) of the permeable mass offiller material, the amount of alloy present in the reservoir couldexceed such sufficient amount so that not only will there be asufficient amount of alloy for complete infiltration, but excess moltenmetal alloy could remain and be attached to the metal matrix compositebody (e.g., a macrocomposite). Thus, when excess molten alloy ispresent, the resulting body will be a complex composite body (e.g., amacrocomposite), wherein an infiltrated ceramic body having a metalmatrix therein will be directly bonded to excess metal remaining in thereservoir.

Further improvements in metal matrix technology can be found in commonlyowned and copending U.S. patent application Ser. No. 07/863,894, filedApr. 6, 1992, now U.S. Pat. No. 5,249,621, which issued on Oct. 5, 1993,which is a continuation of U.S. patent application Ser. No. 07/521,043,filed May 9, 1990, now abandoned, which is a continuation-in-part ofU.S. patent application Ser. No. 07/484,753, filed Feb. 23, 1990, nowabandoned, which is a continuation-in-part of U.S. patent applicationSer. No. 07/432,661, filed Nov. 7, 1989, now abandoned, which is acontinuation-in-part of U.S. patent application Ser. No. 07/416,327,filed Oct. 6, 1989, now abandoned, which is a continuation-in-partapplication of U.S. patent application Ser. No. 07/349,590, filed May 9,1989, now abandoned, which in turn is a continuation-in-part applicationof U.S. patent application Ser. No. 07/269,311, filed Nov. 10, 1988, nowabandoned, all of which were filed in the names of Michael K. Aghajanianet al. and all of which are entitled "A Method of Forming Metal MatrixComposite Bodies By A Spontaneous Infiltration Process, and ProductsProduced Therefrom." According to these Aghajanian et al. patent andpatent applications, spontaneous infiltration of a matrix metal into apermeable mass of filler material or preform is achieved by use of aninfiltration enhancer and/or an infiltration enhancer precursor and/oran infiltrating atmosphere which are in communication with the fillermaterial or preform, at least at some point during the process, whichpermits molten matrix metal to spontaneously infiltrate the fillermaterial or preform. Aghajanian et al. disclose a number of matrixmetal/infiltration enhancer precursor/infiltrating atmosphere systemswhich exhibit spontaneous infiltration. Specifically, Aghajanian et al.disclose that spontaneous infiltration behavior has been observed in thealuminum/magnesium/nitrogen system; the aluminum/strontium/nitrogensystem; the aluminum/zinc/oxygen system; and thealuminum/calcium/nitrogen system. However, it is clear from thedisclosure set forth in the Aghajanian et al. patent and patentapplications that the spontaneous infiltration behavior should occur inother matrix metal/infiltration enhancer precursor/infiltratingatmosphere systems.

Each of the above-discussed Commonly Owned Metal Matrix Patents andPatent Applications describes methods for the production of metal matrixcomposite bodies and novel metal matrix composite bodies which areproduced therefrom. The entire disclosures of all of the foregoingCommonly Owned Metal Matrix Patents and Patent Applications areexpressly incorporated herein by reference.

SUMMARY OF THE INVENTION

A metal matrix composite body can be produced by spontaneouslyinfiltrating a permeable mass of filler material or a preform with amolten matrix metal. The matrix metal in the infiltrated filler materialor preform and/or the filler material or the preform may be modifiedsubstantially contiguously with infiltration and/or may be modified by apost formation process treatment (i.e., may be modified afterinfiltration has been achieved). Such modification results in enhancedor improved properties (e.g., improved mechanical properties especiallyat high temperatures, improved corrosion resistance, improved erosionresistance, etc.) in a formed metal matrix composite. Moreover, metalmatrix composites produced by methods other than a spontaneousinfiltration process also may be treated in accordance with one or bothof the substantially contiguous modification treatment and the postformation process treatment.

To achieve spontaneous infiltration, a permeable mass of filler materialor a preform is contacted with an infiltration enhancer and/orinfiltration enhancer precursor and/or infiltrating atmosphere, at leastat some point during the process, which permits molten matrix metal tospontaneously infiltrate the filler material or preform. In a preferredmethod for achieving spontaneous infiltration, rather than supplying aninfiltration enhancer precursor, an infiltration enhancer may besupplied directly to at least one of the preform and/or matrix metaland/or infiltrating atmosphere. Ultimately, at least during thespontaneous infiltration, the infiltration enhancer should be located inat least a portion of the filler material or preform.

In a preferred embodiment for modifying at least one property of a metalmatrix composite, once infiltration (e.g., spontaneous infiltration)into the preform or filler material has been achieved, but prior to thematrix metal solidifying, the matrix metal in the metal matrix compositebody is modified. Specifically, at least a portion of the matrix metalis caused to be contacted with a second material such as, for example, asecond metal or a precursor to a second metal, which second metal may bein a solid, liquid or vapor state, and which second metal has acomposition different from that of the matrix metal. The second materialmay react with the matrix metal and/or filler material or preform toform one or more desirable reaction products. For example, in the caseof a second material comprising a second metal in a liquid phase, thesecond metal may become interdiffused with the matrix metal, therebyresulting in, for example, the formation of desirable intermetallics dueto a reaction between the matrix metal and/or filler with the secondmetal. Accordingly, in the case of a liquid-phase second metal, it maybe preferable for the second metal to be miscible with (e.g., whendesirable to form intermetallics) the matrix metal. In this embodiment,it should be understood that modification of the metal matrix compositemay occur under processing conditions that are very similar to theprocess conditions used to form the metal matrix composite body (i.e.,achieve metal infiltration) or modification may occur under a set ofconditions which are different from those used to form a metal matrixcomposite. For example, temperature may be increased to cause a reactionto occur which either cannot occur due to thermodynamics or occurs tooslowly due to kinetics.

In another preferred embodiment for modifying at least one property of ametal matrix composite, prior to completing infiltration (e.g.,spontaneous infiltration) of molten matrix metal into a permeable massof filler material or a preform, the composition of the matrix metalwhich continues to infiltrate is changed by adding a second metal (or aprecursor to a second metal) thereto, which has a composition which isdifferent from the matrix metal. For example, once molten matrix metalbegins to infiltrate a permeable mass of filler material or a preform, asecond metal could be added to (e.g., alloyed with) the source of matrixmetal (e.g., a reservoir source of matrix metal). The second metal couldbe any metal which, when combined with the matrix metal, does notadversely affect the infiltration (e.g., spontaneous infiltration) ofmolten matrix metal and modifies the properties of the metal matrixcomposite (e.g., the matrix metal in the metal matrix composite, etc.)in a desired manner (e.g., by forming desirable alloy(s) and/ordesirable reaction product(s) (e.g., intermetallics, etc.)). Again, inthis preferred embodiment, desirable modification of the metal matrixcomposite may occur under conditions which are quite similar to thoseconditions used to manufacture the metal matrix composite (i.e., achievemetal infiltration), or the conditions may be modified (e.g., anincrease in temperature) to permit desirable reaction and/or amounts ofreaction, to occur.

In a further preferred embodiment for modifying at least one property ofa metal matrix composite, at least one of the matrix metal and/or fillermaterial or preform in a metal matrix composite is modifiedsubstantially contiguously with the infiltration (e.g., spontaneousinfiltration) of molten matrix metal into a filler material or preform.In this embodiment, a second material may be admixed at least partially,or substantially completely, with at least a portion of, orsubstantially all of, the filler material or preform, said secondmaterial being reactive with the matrix metal and/or filler material orpreform under a specific set of processing conditions. In some cases,the reaction may occur only during formation of the composite body(i.e., during metal infiltration), whereas in other cases reaction mayoccur only after the formation of the composite body due to a change inprocessing conditions. Additionally, some reactions may occur bothduring the metal infiltration process and during a post-processingtreatment. For example, the second material may comprise a metal whichreacts with molten matrix metal to form desirable reaction products(e.g., alloys or intermetallics) which improve, for example, the hightemperature strength, corrosion resistance, erosion resistance,electrical conductivity, etc., of the metal matrix composite.Additionally, the second metal may react with, for example, the fillermaterial or preform, to form one or more desirable reaction productoxide, carbide(s), nitride(s), boride(s), etc. Moreover, the secondmaterial may comprise a precursor to a second metal which reacts with,for example, the matrix metal to liberate the second metal which thenmay behave in a manner similar to that discussed immediately above.Still further, it may be possible to employ as second materials in thisembodiment, materials which would otherwise react during theinfiltration process. For example, by coating such second materials withone or more other materials (for example, by a sol-gel process, a CVDprocess, etc.) the coating could protect the substrate second materialduring the infiltration process, but at an elevated temperature such asmight be employed in a post-formation treatment process, the coatingcould then become non-protective and thereby permit the substrate secondmaterial to react with at least one of the filler material or matrixmetal contained within the composite body to form one or more desireablealloy(s) and/or reaction product(s).

In another preferred embodiment, infiltration (e.g., spontaneousinfiltration) is carried out for a time which is not sufficient topermit molten matrix metal to embed completely the filler material orpreform (e.g., at least some porosity is created or formed in the fillermaterial or preform). A second metal which is different in compositionfrom the matrix metal may then be contacted with a surface of the metalmatrix composite body which has not undergone complete infiltration. Thesecond metal then infiltrates into the porosity of the metal matrixcomposite, (e.g., the second metal may alloy with the infiltrated matrixmetal and provide a sufficient quantity of alloyed matrix metal to fillsubstantially completely the porosity in the filler material orpreform). Moreover, such filling-in of the porosity should occur at atemperature at or above the liquidus temperature of the matrix metal(and/or alloy of matrix metal and second metal). Accordingly, the metalmatrix composite body will be modified by the filling-in of an alloy ofmatrix metal and second metal into the porosity of a filler material orpreform. Such "filling-in" may permit various desirable alloys and/orreaction products to be formed, as discussed above herein.

In another preferred embodiment, a second material such as, for example,a second metal or a precursor to a second metal, which may be in asolid, liquid or vapor-phase, and having a composition which isdifferent from the matrix metal that has infiltrated a filler materialor preform, can be contacted with at least a portion of a substantiallycompletely infiltrated filler material or preform, and said secondmaterial reacts with at least one of the matrix metal and/or fillermaterial or preform. Specifically, in a preferred method, a second metalor a precursor to a second metal can be transported by matrix metal intocontact with the filler material or preform and/or may contact thefiller material or preform directly, and thereby react with the fillermaterial or preform to form one or more desirable alloys and/or reactionproducts. In this preferred method, a reaction product(s) can be formedwhich undergoes a volumetric expansion relative to the original fillermaterial or preform. Such reaction product typically is formed when thematrix metal is at, above, or slightly below the liquidus temperaturewhich results in matrix metal being displaced from the metal matrixcomposite body. Accordingly, depending upon the amount of reactionproduct formed, an overall volume percent of matrix metal in the metalmatrix composite body is reduced. For example, the formation of reactionproduct could be limited to a surface area of the metal matrixcomposite, thus forming a reaction product surface on a metal matrixcomposite substrate. Moreover, the formation of a reaction product isnot limited to metal matrix composite bodies produced according to aspontaneous infiltration technique. It is conceivable that the formationof reaction product in any system which involves a conversion of matrixmetal and/or filler material or preform to a reaction product which thendisplaces matrix metal can produce desirable results.

In a further preferred embodiment of the present invention, the matrixmetal of a formed metal matrix composite may be modified by providing atleast one grain refiner within at least a portion of the filler materialor preform and/or matrix metal. Specifically, a grain refiner maycomprise any material (e.g., metal, oxide, nitride, carbide, etc.)which, under the processing conditions, initiates preferentialheterogeneous nucleation of at least one phase, other than the matrixmetal phase, within the metallic constituent of the formed metal matrixcomposite, thus modifying the matrix metal and properties of the metalmatrix composite. The grain refiners are typically solid under theprocessing conditions. In a particularly preferred embodiment, grainrefiners may be created by, for example, ball milling a filler materialto break off, for example, edges of the filler particulate. Under theprocessing conditions, these very small filler particles act asnucleation sites for the precipitation of at least one phase, other thanthe matrix metal phase, within the metallic constituent of the metalmatrix composite body. Moreover, rather than creating grain refiners byball milling the filler material, grain refiners may be added to atleast a portion of the filler material or preform and/or matrix metal inorder to achieve similar precipitation in the metallic constituent. Forexample, in the case of an aluminum matrix metal, suitable grainmodifiers may comprise alumina, titanium diboride, zirconium diboride,titanium aluminides, aluminum borides, manganese, and the like, andcombinations thereof.

It is noted that this application, with regard to the spontaneousinfiltration technique, discusses primarily aluminum matrix metalswhich, at some point during the formation of the metal matrix compositebody, are contacted with magnesium, which functions as the infiltrationenhancer precursor, in the presence of nitrogen, which functions as theinfiltrating atmosphere. Thus, the matrix metal/infiltration enhancerprecursor/infiltrating atmosphere system of aluminum/magnesium/nitrogenexhibits spontaneous infiltration. However, other matrixmetal/infiltration enhancer precursor/infiltrating atmosphere systemsmay also behave in a manner similar to the systemaluminum/magnesium/nitrogen. For example, similar spontaneousinfiltration behavior has been observed in thealuminum/strontium/nitrogen system; the aluminum/zinc/oxygen system; andthe aluminum/calcium/nitrogen system. Accordingly, even though thealuminum/magnesium/nitrogen system is discussed primarily herein, itshould be understood that other matrix metal/infiltration enhancerprecursor/infiltrating atmosphere systems may behave in a similar mannerand are intended to be encompassed by the invention.

When the matrix metal comprises an aluminum alloy, the aluminum alloy iscontacted with a preform comprising a filler material (e.g., alumina orsilicon carbide) or a filler material, said filler material or preformhaving admixed therewith, and/or at some point during the process beingexposed to, magnesium. Moreover, in a preferred embodiment, the aluminumalloy and/or preform or filler material are contained in a nitrogenatmosphere for at least a portion of the process. The preform will bespontaneously infiltrated and the extent or rate of spontaneousinfiltration and formation of metal matrix will vary with a given set ofprocess conditions including, for example, the concentration ofmagnesium provided to the system (e.g., in the aluminum alloy and/or inthe filler material or preform and/or in the infiltrating atmosphere),the size and/or composition of the particles in the preform or fillermaterial, the concentration of nitrogen in the infiltrating atmosphere,the time permitted for infiltration, and/or the temperature at whichinfiltration occurs. Spontaneous infiltration typically occurs to anextent sufficient to embed substantially completely the preform orfiller material.

Definitions

"Aluminum", as used herein, means and includes essentially pure metal(e.g., a relatively pure, commercially available unalloyed aluminum) orother grades of metal and metal alloys such as the commerciallyavailable metals having impurities and/or alloying constituents such asiron, silicon, copper, magnesium, manganese, chromium, zinc, etc.,therein. An aluminum alloy for purposes of this definition is an alloyor intermetallic compound in which aluminum is the major constituent.

"Balance Non-Oxidizing Gas", as used herein, means that any gas presentin addition to the primary gas comprising the infiltrating atmosphere,is either an inert gas or a reducing gas which is substantiallynon-reactive with the matrix metal under the process conditions. Anyoxidizing gas which may be present as an impurity in the gas(es) usedshould be insufficient to oxidize the matrix metal to any substantialextent under the process conditions.

"Barrier" or "barrier means", as used herein, means any suitable meanswhich interferes, inhibits, prevents or terminates the migration,movement, or the like, of molten matrix metal beyond a surface boundaryof a permeable mass of filler material or preform, where such surfaceboundary is defined by said barrier means. Suitable barrier means may beany such material, compound, element, composition, or the like, which,under the process conditions, maintains some integrity and is notsubstantially volatile (i.e., the barrier material does not volatilizeto such an extent that it is rendered non-functional as a barrier).

Further, suitable "barrier means" includes materials which aresubstantially non-wettable by the migrating molten matrix metal underthe process conditions employed. A barrier of this type appears toexhibit substantially little or no affinity for the molten matrix metal,and movement beyond the defined surface boundary of the mass of fillermaterial or preform is prevented or inhibited by the barrier means. Thebarrier reduces any final machining or grinding that may be required anddefines at least a portion of the surface of the resulting metal matrixcomposite product. The barrier may in certain cases be permeable orporous, or rendered permeable by, for example, drilling holes orpuncturing the barrier, to permit gas to contact the molten matrixmetal.

"Carcass" or "Carcass of Matrix Metal", as used herein, refers to any ofthe original body of matrix metal remaining which has not been consumedduring formation of the metal matrix composite body, and typically, ifallowed to cool, remains in at least partial contact with the metalmatrix composite body which has been formed. It should be understoodthat the carcass may also include a second or foreign metal therein.

"Different" means a material (e.g., a metal) which does not contain, asa primary constituent, the same material as a comparative material(e.g., in the case of a metal, if the primary constituent of a matrixmetal is aluminum, the "different" metal could have a primaryconstituent of, for example, nickel).

"Filler", as used herein, is intended to include either singleconstituents or mixtures of constituents which are substantiallynon-reactive with and/or of limited solubility in the matrix metal andmay be single or multi-phase. Fillers may be provided in a wide varietyof forms, such as powders, flakes, platelets, microspheres, whiskers,bubbles, fibers, particulates, fiber mats, chopped fibers, spheres,pellets, tubules, refractory cloths, etc., and may be either dense orporous. "Filler" may also include ceramic fillers, such as alumina orsilicon carbide as fibers, chopped fibers, particulates, whiskers,bubbles, spheres, fiber mats, pellets, tubules, refractory cloths, orthe like, and coated fillers such as carbon fibers coated with aluminaor silicon carbide to protect the carbon from attack, for example, by amolten aluminum matrix metal. Fillers may also include metals.

"Grain Refiner", as used herein, means any material (e.g., metal, oxide,carbide, nitride, etc.) which, under the process conditions, initiatespreferential nucleation of at least one phase, other than the matrixmetal phase, within a metallic constituent of a metal matrix compositebody and wherein such nucleation modifies the matrix metal (e.g., altersthe morphology) and thus the properties of the metal matrix composite.Grain refiners can comprise a solid material under the processingconditions.

"Infiltrating Atmosphere", as used herein, means that atmosphere whichis present which interacts with the matrix metal and/or preform (orfiller material) and/or infiltration enhancer precursor and/orinfiltration enhancer and permits or enhances spontaneous infiltrationof the matrix metal to occur.

"Infiltration Enhancer", as used herein, means a material which promotesor assists in the spontaneous infiltration of a matrix metal into afiller material or preform. An infiltration enhancer may be formed as,for example, (1) a reaction product of an infiltration enhancerprecursor with an infiltrating atmosphere, (2) a reaction product of theinfiltration enhancer precursor and the matrix metal or (3) a reactionproduct of the infiltration enhancer precursor and the filler materialor preform. Moreover, the infiltration enhancer may be supplied directlyto at least one of the preform and/or matrix metal and/or infiltratingatmosphere and function in a substantially similar manner to aninfiltration enhancer which has formed from a reaction between aninfiltration enhancer precursor and another species. Ultimately, atleast during the spontaneous infiltration, the infiltration enhancershould be located in at least a portion of the filler material orpreform to achieve spontaneous infiltration. The infiltration enhancermay be at least partially reducible by the matrix metal.

"Infiltration Enhancer Precursor" or "Precursor to the InfiltrationEnhancer", as used herein, means a material which when used incombination with (1) the matrix metal, (2) the preform or fillermaterial and/or (3) an infiltrating atmosphere forms an infiltrationenhancer which induces or assists the matrix metal to spontaneouslyinfiltrate the filler material or preform. Without wishing to be boundby any particular theory or explanation, it appears as though it may benecessary for the precursor to the infiltration enhancer to be capableof being positioned, located or transportable to a location whichpermits the infiltration enhancer precursor to interact with theinfiltrating atmosphere and/or the preform or filler material and/or thematrix metal. For example, in some matrix metal/infiltration enhancerprecursor/infiltrating atmosphere systems, it is desirable for theinfiltration enhancer precursor to volatilize at, near, or in somecases, even somewhat above the temperature at which the matrix metalbecomes molten. Such volatilization may lead to: (1) a reaction of theinfiltration enhancer precursor with the infiltrating atmosphere to forma gaseous species which enhances wetting of the filler material orpreform by the matrix metal; and/or (2) a reaction of the infiltrationenhancer precursor with the infiltrating atmosphere to form a solid,liquid or gaseous infiltration enhancer in at least a portion of thefiller material or preform which enhances wetting; and/or (3) a reactionof the infiltration enhancer precursor within the filler material orpreform (such as by reacting with the filler material itself and/or theinfiltrating matrix metal) which forms a solid, liquid or gaseousinfiltration enhancer in at least a portion of the filler material orpreform which enhances wetting.

"Interdiffusion" or "Interdiffused", as used herein, means that thereoccurs at least partial contact or mixing of a matrix metal with asecond or different metal, to result in a new desirable alloy and/orintermetallic.

"Matrix Metal" or "Matrix Metal Alloy", as used herein, means that metalwhich is utilized to form a metal matrix composite (e.g., beforeinfiltration) and/or that metal which is intermingled with a fillermaterial to form a metal matrix composite body (e.g., afterinfiltration). When a specified metal is mentioned as the matrix metal,it should be understood that such matrix metal includes that metal as anessentially pure metal, a commercially available metal having impuritiesand/or alloying constituents therein, an intermetallic compound or analloy in which that metal is the major or predominant constituent.

"Matrix Metal/Infiltration Enhancer Precursor/Infiltrating AtmosphereSystem" or "Spontaneous System", as used herein, refers to thatcombination of materials which exhibit spontaneous infiltration into apreform or filler material. It should be understood that whenever a "/"appears between an exemplary matrix metal, infiltration enhancerprecursor and infiltrating atmosphere that the "/" is used to designatea system or combination of materials which, when combined in aparticular manner, exhibits spontaneous infiltration into a preform orfiller material.

"Metal Matrix Composite" or "MMC", as used herein, means a materialcomprising a two- or three-dimensionally interconnected alloy or matrixmetal which has embedded a preform or filler material. The matrix metalmay include various alloying elements to provide specifically desiredmechanical and physical properties in the resulting composite.

"Nonreactive Vessel for Housing Matrix Metal" means any vessel which canhouse or contain a filler material (or preform) and/or molten matrixmetal under the process conditions and not react with the matrix and/orthe infiltrating atmosphere and/or infiltration enhancer precursorand/or filler material or preform in a manner which would besignificantly detrimental to the spontaneous infiltration mechanism.

"Preform" or "Permeable Preform", as used herein, means a porous mass offiller or filler material which is manufactured with at least onesurface boundary which essentially defines a boundary for infiltratingmatrix metal, such mass retaining sufficient shape integrity and greenstrength to provide dimensional fidelity prior to being infiltrated bythe matrix metal. The mass should be sufficiently porous to accommodatespontaneous infiltration of the matrix metal thereinto. A preformtypically comprises a bonded array or arrangement of filler, eitherhomogeneous or heterogeneous, and may be comprised of any suitablematerial (e.g., ceramic and/or metal particulates, powders, fibers,whiskers, etc., and any combination thereof). A preform may exist eithersingularly or as an assemblage.

"Reaction Product", as used herein, means the product of a reactionbetween: (1) a second (or different) metal (or a precursor to a secondmetal); or (2) a second material with at least one of: (1) a fillermaterial; (2) a preform; (3) a matrix metal; and/or (4) anotherdifferent second metal or second material. "Reaction Product" shouldalso be understood as including intermetallic compounds which form as aresult of the above-described reaction(s).

"Reservoir", as used herein, means a separate body of matrix metalpositioned relative to a mass of filler or a preform so that, when themetal is molten, it may flow to replenish, or in some cases to initiallyprovide and subsequently replenish, that portion, segment or source ofmatrix metal which is in contact with the filler or preform.

"Second Material", as used herein, means a material which may, undercertain processing conditions, react with at least one of the matrixmetal, filler material and/or preform to produce at least one desirablereaction product in the metal matrix composite, thereby modifying atleast one property of the metal matrix composite in a desirable manner.The second material may be admixed in the filler material or preformprior to infiltration of the filler material or preform by the matrixmetal. Alternatively, the second material may be provided in the sourceof matrix metal itself either before, during or after infiltration ofthe matrix metal into the permeable mass. Still further, the secondmaterial may be provided as precursor to a second material whichsubsequently reacts, decomposes, changes state or phase or is otherwisemodified to form the second material. The second material may comprise ametal (in which case the "second metal" may be referred to in the textas a "second metal"), a precursor to a metal, a non-metal which reactsto form other non-metals, etc.

"Spontaneous Infiltration", as used herein, means the infiltration ofmatrix metal into the permeable mass of filler or preform occurs withoutrequirement for the application of pressure or vacuum (whetherexternally applied or internally created).

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic cross-sectional view of a setup for making a metalmatrix composite body in accordance with Example 1;

FIG. 2 is a schematic cross-sectional view of a setup for making a metalmatrix composite body in accordance with Example 2;

FIG. 3 is a schematic cross-sectional view of a setup for modifying aformed metal matrix composite body in accordance with Example 2;

FIG. 4 is a secondary electron image photograph at about 1000×magnification, of the microstructure of the metal matrix composite bodyas treated in accordance with Example 2;

FIGS. 5a, 5b, 5c and 5d are comparative photographs taken at about 400×magnification of the microstructures of the metal matrix compositebodies formed in Example 3;

FIG. 6 is a photomicrograph at about 400× magnification of the metalmatrix composite body as modified in Example 5;

FIG. 7 is a cross-sectional schematic view of the lay-up employed infabricating the metal matrix composite bodies described in Example 6;

FIG. 8 is a graph showing the hardness of a metal matrix composite bodyas a function of the percentage of a second material in the preform;

FIG. 9 is a cross-sectional schematic view of the lay-up employed infabricating the metal matrix composite bodies of Example 7;

FIG. 10 is a cross-sectional schematic view of the lay-up employed infabricating the metal matrix composite bodies of Example 8;

FIGS. 11a and 11b are optical photomicrographs at about 400×magnification of the samples, respectively, of the metal matrixcomposite body Example 8 as-infiltrated and post-process-treated,respectively;

FIG. 12 is a cross-sectional schematic view of the lay-up employed infabricating the metal matrix composite body of Example 9; and

FIG. 13 is a cross-sectional schematic view of the lay-up employed inpost-treating the formed metal matrix composite bodies of Examples 9 and10.

DETAILED DESCRIPTION OF THE INVENTION AND PREFERRED EMBODIMENTS

The present invention relates to forming a metal matrix composite bodyby, for example, spontaneously infiltrating a permeable mass of fillermaterial or preform with molten matrix metal and during and/orsubsequent to said forming step modifying at least a portion of themetal matrix composite body.

A metal matrix composite body can be produced by, for example,spontaneously infiltrating a permeable mass of filler material or apreform with a molten matrix metal. The matrix metal in the infiltratedfiller material or preform and/or the filler material or the preform maybe modified substantially contiguously with infiltration; and/or may bemodified by a post formation process treatment (i.e., may be modifiedafter infiltration has been achieved). Such modification results inenhanced or improved properties (e.g., improved mechanical properties,improved corrosion resistance, improved erosion resistance, etc.) in aformed metal matrix composite. Moreover, metal matrix compositesproduced by methods other than a spontaneous infiltration process alsomay be treated in accordance with either or both of a substantiallycontiguous modification treatment or a post formation process treatment.

To achieve spontaneous infiltration, a permeable mass of filler materialor a preform is contacted with an infiltration enhancer and/orinfiltration enhancer precursor and/or infiltrating atmosphere, at leastat some point during the process, which permits molten matrix metal tospontaneously infiltrate the filler material or preform. In a preferredmethod for achieving spontaneous infiltration, rather than supplying aninfiltration enhancer precursor, an infiltration enhancer may besupplied directly to at least one of the preform and/or matrix metaland/or infiltrating atmosphere. Ultimately, at least during thespontaneous infiltration, the infiltration enhancer should be located inat least a portion of the filler material or preform.

In a preferred embodiment for modifying at least one property of a metalmatrix composite, once infiltration (e.g., spontaneous infiltration)into the preform or filler material has been achieved, but prior to thematrix metal solidifying, the matrix metal in the metal matrix compositebody is modified. Specifically, at least a portion of the matrix metalis caused to be contacted with a second material such as, for example, asecond metal or a precursor to a second metal, which second metal may bein a solid, liquid or vapor phase, and which second metal has acomposition different from that of the matrix metal. The second materialmay react with the matrix metal and/or filler material or preform toform one or more desirable reaction products. For example, in the caseof a second material comprising a second metal in a liquid phase, thesecond metal may become interdiffused with the matrix metal, therebyresulting in, for example, the formation of desirable intermetallics dueto a reaction between the matrix metal and/or filler material with thesecond metal. Accordingly, in the case of a liquid-phase second metal,it may be preferable for the second metal to be miscible with (e.g.,when desirable to form intermetallics) the matrix metal. In thisembodiment, it should be understood that modification of the metalmatrix composite may occur under processing conditions that are verysimilar to the process conditions used to form the metal matrixcomposite body (i.e., achieve metal infiltration) or modification mayoccur under a set of conditions which are different from those used toform a metal matrix composite. For example, temperature may be increasedto permit a reaction to occur which either cannot occur due tothermodynamics or occurs too slowly due to kinetics.

Moreover, under appropriate reaction conditions, a metal matrixcomposite may be contacted with a second metal or a precursor to asecond metal in a vapor phase, in order to modify the composition of thematrix metal. In certain instances, it may be necessary to tailor thereaction conditions so that the thermodynamics and/or kinetics ofreaction between, for example, the matrix metal and the second metal, donot result in matrix metal being exuded from the formed composite body.

In the practice of a preferred embodiment, a stainless steel vessel canbe utilized, said stainless steel vessel comprising a container having abottom plate sealed into position by an annular copper gasket. Thestainless steel container preferably contains a refractory support. Thestainless steel container may have therein an alumina filler, such as 90grit, 38 Alundum supplied by Norton Co., with about 50% by volume of apowdered second metal, said powdered second metal being different fromthe matrix metal. In this embodiment, the matrix metal alloy may beplaced on the filler material in the stainless steel can. The matrixmetal preferably comprises an aluminum alloy with about 5 percent byweight Mg. In order to spontaneously infiltrate matrix metal into thefiller material, the stainless steel vessel should be heated to about750°-1100° C. in an atmosphere of a nitrogen-containing gas. As thematrix metal spontaneously infiltrates into the permeable filler, itcontacts the second metal contained in the filler (e.g., nickel, copper,silicon, magnesium, etc.). The aluminum in the matrix metal may reactwith, for example, the nickel in the filler to form an intermetallicsuch as a nickel aluminide in the channels through which the matrixmetal passed in order to spontaneously infiltrate the filler. The degreeto which any reaction occurs depends upon a number of factors includingtemperature, the length of exposure at this temperature, and/or themiscibility of the molten metals.

In another preferred embodiment for modifying at least one property of ametal matrix composite, prior to completing infiltration (e.g.,spontaneous infiltration) of molten matrix metal into a permeable massof filler material or a preform, the composition of the matrix metalwhich continues to infiltrate is changed by adding a second metal (or aprecursor to a second metal) thereto, which has a composition which isdifferent from the matrix metal. For example, once molten matrix metalbegins to infiltrate a permeable mass of filler material or a preform, asecond metal could be added to (e.g., alloyed with) the source of matrixmetal (e.g., a reservoir source of matrix metal). The second metal couldbe any metal which, when combined with the matrix metal, does notadversely affect the infiltration (e.g., spontaneous infiltration) ofmolten matrix metal and modifies the properties of the metal matrixcomposite (e.g., the matrix metal in the metal matrix composite, etc.)in a desired manner (e.g., by forming desirable alloy(s) and/ordesirable reaction product(s) (e.g., intermetallics, etc.)). Again, inthis preferred embodiment, desirable modification of the metal matrixcomposite may occur under conditions which are quite similar to thoseconditions used to manufacture the metal matrix composite (i.e., duringmetal infiltration), or the conditions may be modified (e.g., anincrease in temperature) to permit desirable reactions and/or amounts ofreactions to occur.

In the practice of a preferred embodiment, an aluminum metal matrixcomposite body is formed under appropriate reaction conditions byinfiltrating a filler material or preform (e.g., silicon carbide) with amolten aluminum matrix metal. Upon substantially complete infiltrationof the filler material or preform with molten matrix metal, or upon atleast partial infiltration of the filler material or preform with moltenaluminum metal, a source of a second metal, for example, silicon, may beadded in any appropriate manner to the molten aluminum matrix metal tomodify the metallic constituent of the resultant metal matrix compositebody.

In a further preferred embodiment for modifying at least one property ofa metal matrix composite, at least one of the matrix metal and/or fillermaterial or preform in a metal matrix composite is at least brought intocontact with the second material substantially contiguously with theinfiltration (e.g., spontaneous infiltration) of molten matrix metalinto a filler material or preform. In this embodiment, a second materialmay be admixed at least partially, or substantially completely, with atleast a portion of, or substantially all of, the filler material orpreform, said second material being reactive with the matrix metaland/or filler material or preform under a specific set of processingconditions.

In some cases, the reaction may occur only during formation of thecomposite body (i.e., during metal infiltration), whereas in other casesreaction may occur only after the formation of the composite body due toa change in processing conditions. In other instances, some reaction mayoccur both during the metal infiltration process and during apostprocessing treatment.

The second material may comprise a metal which reacts with molten matrixmetal to form, for example, desirable intermetallics and/or otherreaction products (e.g., oxides, carbides nitrides, borides, etc.) whichimprove, for example, the high temperature strength, corrosionresistance, erosion resistance, electrical conductivity (orresistivity), hardness, etc., of the metal matrix composite.Additionally, the second metal may react with, for example, the fillermaterial or preform, to form one or more desirable reaction productintermetallics, oxides, carbides, nitrides, borides, etc.

Moreover, the second material may comprise a precursor to a second metalwhich reacts with, for example, the matrix metal to liberate the secondmetal which then may behave in a manner similar to that discussedimmediately above. For example, in the case of an aluminum matrix metal,a second material comprising a precursor to a second metal such as, forexample, copper oxide, iron oxide or nickel oxide (i.e., oxides whichtend to be less stable than alumina under the reaction conditions) maybe mixed into the filler material. Under the processing conditions, thesecond material would react with, for example, the molten aluminummatrix metal to liberate the second metal into the matrix metal and, inaddition, result in, for example, an aluminum oxide reinforcement phasebeing formed within the composite body. Still further, under theappropriate process conditions, the liberated second metal may reactwith, for example, the aluminum matrix metal to form one or moredesirable intermetallic compounds.

Another example of a second material comprising a precursor to a secondmetal, in the case of an aluminum matrix metal, is a second materialcomprising at least one nitride such as silicon nitride (i.e., nitrideswhich tend to be less stable than aluminum nitride under the reactionconditions) or precursors thereof (e.g., preceramic polymers such asCERASET™ SN preceramic polymer, available from Lanxide Corporation,Newark, Del. which may be placed into the filler material or preform.Under the processing conditions, such nitride materials could reactwith, for example, molten aluminum matrix metal which would liberate atleast one second metal (e.g., silicon) into the matrix metal and, inaddition, result in an aluminum nitride reinforcement phase beingpresent within at least a portion of the composite body.

In yet another example utilizing the case of an aluminum matrix metal, asecond material comprising neither a metal nor a precursor to a metal,such as boron carbide, may be placed into the filler material orpreform. Under the processing conditions, such boron carbide could reactwith, for example, molten aluminum matrix metal to form a plurality ofreaction products such as aluminum borides, aluminum borocarbides, etc.In a particularly preferred embodiment for applications of the resultingcomposite material requiring extreme hardness, the filler material orpreform comprises both silicon nitride and a carbon source such aselemental carbon or a carbon-containing compound such as boron carbide.Without wishing to be bound by any particular theory or explanation,under the processing conditions utilizing an aluminum matrix metal, thesilicon nitride may react with the infiltrated molten aluminum, as shownby Equation 1 below, to produce aluminum nitride plus reduced silicon.The carbon source may then react with the silicon metal to producesilicon carbide. The net reaction may be described by Equation 2 below.

    (1) 4 Al+Si.sub.3 N.sub.4 →4 AlN+3 Si

    (2) 4 Al+Si.sub.3 N.sub.4 +3 C→4 AlN+3 SiC

Further examples of desirable second material additions which can beused in combination with an aluminum matrix metal include: titaniumdioxide (TiO₂), which may result in a plurality of reaction productsbeing formed including Al₂ O₃ and Al--Ti intermetallics; mixtures ofmaterials including Si₃ N₄ with MoO₃, which may result in the formationof, for example, AlN, Al₂ O₃, MoSi₂, etc.; mixtures of materialsincluding SiO₂ with MoO₃, which may result in the formation of, forexample, Al₂ O₃ and MoSi₂ ; and TiO₂ with Si₃ N₄, which may result inthe formation of Al₂ O₃, AlN, TiN, Al--Ti intermetallics, etc. Furtherexamples of desirable precursors to second materials which could beadded to a permeable mass to use in combination with an aluminum matrixmetal include mullite (Al₆ Si₂ O₁₃) and aluminum titanate (Al₂ TiO₅).Under the appropriate processing conditions, mullite may decompose to asubstance comprising as a second material, silicon dioxide (SiO₂).Likewise, aluminum titanate may decompose to a substance comprising as asecond material, titanium dioxide (TiO₂). Various other combinations ofmaterials should be readily apparent to those of ordinary skill in theart. Specifically, any combination of materials which when subjected tothe appropriate reaction conditions may react to form reaction productswhich desirably impact a composite body are also the subject of thispreferred embodiment.

When a second material is added to a filler material or preform, thevolume percent of second material added can vary over a wide rangedepending on a number of different considerations. For example, if themetal matrix composite formation conditions had an upper limit on theamount of filler material that could be infiltrated successfully by amatrix metal, however such upper limit did not permit the metal matrixcomposite body to function in a commercially acceptable manner, then anappropriate amount of a second material could be added to the fillermaterial or preform to result in an effective increase in the volumepercent of reinforcement in a metal matrix composite body due to theformation of reaction product. Moreover, if a substantial amount of anappropriate second material was added to a filler material or preform,and a sufficient amount of, for example, matrix metal was present toreact with such second material, the amount of reaction product formedcould cause the reaction product to form an at least partially, orsubstantially completely, interconnected reaction product matrix. Stillfurther, in some cases it may be desirable for only a small amount ofreaction product to be formed. Accordingly, only a small amount ofsecond material would need to be added to the filler material orpreform.

As discussed above herein, in some cases it may be desirable to utilizea second material which is substantially non-reactive with the matrixmetal and/or filler material under the infiltration conditions, butreacts readily when one or more of the processing conditions required toachieve infiltration are changed. For example, the second material maycomprise a second metal which may be admixed with the filler material orpreform prior to infiltration or may be added to or admixed with thesource of matrix metal either prior to, during or subsequent toinfiltration. In this manner, a metal matrix composite body may befabricated which comprises the second metal in substantially unreactedform. Upon changing the process conditions (for example, increasing thetemperature above the temperature employed for infiltration), however,the second metal within the formed metal matrix composite body may reactwith at least one of the filler material or matrix metal within thecomposite body to form a reaction product which may include, forexample, one or more intermetallic compounds. Similarly, the secondmaterial may comprise a precursor to a second metal or other materialssuch as the above-described boron carbide, etc., admixed at leastpartially or substantially completely within the filler material orpreform which is to be infiltrated. The filler material or preformcontaining this second material may then be infiltrated with moltenmatrix metal without significant reaction of the second material to forma metal matrix composite body comprising the substantially unreactedsecond material. The formed composite body may then be raised to atemperature above the infiltration temperature to react the secondmaterial contained within the metal matrix composite body with at leastone of the filler material and the matrix metal contained within thecomposite to form at least one reaction product which may comprise, forexample, one or more intermetallic or ceramic compounds. Furthermore, itmay be possible to employ as second materials in this embodiment,materials which would otherwise react during the infiltration process ormay react too rapidly during post-treatment processing. For example, bycoating such second materials with one or more other materials (forexample, by a sol-gel process, chemical vapor deposition or any otherappropriate means) such as, for example, with a colloidal ceramic (e.g.,colloidal alumina) the coating could protect the substrate secondmaterial during the infiltration process, but at an elevated temperaturesuch as might be employed in a post-formation treatment process, thecoating could then become non-protective and thereby permit thesubstrate second material to react with at least one of the fillermaterial or matrix metal contained within the composite body to form oneor more desireable alloy(s) and/or reaction product(s). Through the useof such a coating process, the number of desirable second materialsavailable for use in the process is expanded.

In the case of spontaneous infiltration, useful amounts of secondmaterial added to a filler material prior to infiltration typicallyrange between at least about 2 percent by volume of the filler materialor preform to as much as about 90 percent or more by volume.

Still further, in some cases the second material may be the onlymaterial that is to be infiltrated. In these instances, the secondmaterial, in some cases, may be at least partially reactive during theinfiltration conditions, but not fully react with, for example, matrixmetal until the infiltrated material is subjected to higher temperaturesand/or longer times. Accordingly, both thermodynamics and kinetics needto be considered when utilizing a second material in this manner. Byselecting appropriate combinations of matrix metal, second material andprocessing conditions, desirable metal matrix composite bodies may beproduced.

In certain cases where a reaction product is formed subsequent tosubstantially complete infiltration of a filler material mixture orpreform, the reaction product, in combination with any liberatedcomponents (e.g., metal(s)) into the matrix metal, may actually occupy asmaller volume than the materials which reacted to form the reactionproduct(s). In such cases, it may be desirable to contact the formedmetal matrix composite with a source of additional metal. Specifically,if porosity was not a desired constituent in the formed metal matrixcomposite, then the source of additional metal could flow into thealready existing metallic constituent and/or resupply or replenish anymetallic constituent used to form reaction product. The source ofadditional metal could have a composition which was substantially thesame as, or substantially different from, the composition of themetallic constituent of the formed metal matrix composite body. It hasbeen discovered that the amount of additional metal supplied and theprecise location of the additional matrix metal can favorably influencethe resultant metal matrix composite body, as shown in some of theExamples appended hereto. It has also been discovered that when it isnecessary to employ an additional source of metal which is substantiallydifferent from the existing or desired metallic constituent of theformed metal matrix composite body to avoid porosity formation, forexample, during a post-process treatment, it may be particularlydesirable to use only as much additional metal as is required toaccomplish the desired objective. Specifically, an excessive quantity ofadditional metal may modify the chemical composition of the metallicmatrix beyond what is desired.

Additionally, it has been discovered that the second material maycomprise a material such as, for example, colloidal alumina, whichfacilitates the binding or rigidizing a preform or filler material, aswell as reacting with at least one of the constituents of the matrixmetal and/or the filler material or preform to modify the metallicconstituent of the metal matrix composite.

In another preferred embodiment, infiltration (e.g., spontaneousinfiltration) is carried out for a time which is not sufficient topermit molten matrix metal to embed completely the filler material orpreform (e.g., at least some porosity is created or formed in the fillermaterial or preform). Alternatively, a substantially completelyinfiltrated metal matrix composite body may be contacted with, forexample, a leachant, to remove a desired amount of matrix metal, thuscreating at least some porosity in the filler material or preform. Asecond metal which is different in composition from the matrix metal maythen be contacted with a surface of the metal matrix composite bodywhich has not undergone complete infiltration. The second metal theninfiltrates into the porosity of the metal matrix composite (e.g., thesecond metal may alloy with the infiltrated matrix metal and provide asufficient quantity of alloyed matrix metal to fill substantiallycompletely the porosity in the filler material or preform). Moreover,such filling-in of the porosity should occur at a temperature at orabove the liquidus temperature of the matrix metal (and/or alloy ofmatrix metal and second metal). Accordingly, the metal matrix compositebody will be modified by the filling-in of an alloy of matrix metal andsecond metal into the porosity of a filler material or preform. Such"filling-in" may permit various desirable alloys and/or reactionproducts to be formed, as described above herein.

In another preferred embodiment, a second material such as, for example,a second metal or a precursor to a second metal, which may be in asolid, liquid or vapor phase, and having a composition which isdifferent from the matrix metal that has infiltrated a filler materialor preform, can be contacted with at least a portion of a substantiallycompletely infiltrated filler material or preform, and said secondmaterial reacts with at least one of the matrix metal and/or fillermaterial or preform. Specifically, in a preferred method, a second metalor a precursor to a second metal can be transported by matrix metal intocontact with the filler material or preform, and/or may contact thefiller material or preform directly, and thereby react with the fillermaterial or preform to form one or more desirable alloys and/or reactionproducts. In this preferred method, a reaction product(s) can be formedwhich undergoes a volumetric expansion relative to the original fillermaterial or preform. Such reaction product typically is formed when thematrix metal is at, above, or slightly below the liquidus temperature,which results in matrix metal being displaced from the metal matrixcomposite body. For example, in the case of an aluminum matrix metal andan alumina filler material or preform, the metal content of the metalmatrix composite may be reduced by contacting the composite body with asecond metal (or a precursor to a second metal), such as magnesium,lithium, strontium, barium, calcium, or the like, under appropriateprocessing conditions, to form a reaction product which displaces thematrix metal from the composite body in favor of additionalreinforcement. Accordingly, depending upon the amount of reactionproduct formed, an overall volume percent of matrix metal in the metalmatrix composite body is reduced. For example, the formation of reactionproduct could be limited to a surface area of the metal matrixcomposite, thus forming a reaction product surface on a metal matrixcomposite substrate. Moreover, the formation of a reaction product isnot limited to metal matrix composite bodies produced according to aspontaneous infiltration technique. It is conceivable that the formationof reaction product in any system which involves a conversion of matrixmetal and/or filler material or preform to a reaction product, whichthen displaces the matrix metal, could produce desirable results.

This application discloses primarily methods of modifying the propertiesof metal matrix composite bodies produced according to a spontaneousinfiltration process. However, from the text, it should be understoodthat some of the modification methods may also be applicable to metalmatrix composites made by alternative methods.

Moreover, in any of the above discussed alteration methods, the amountor portion of metal matrix composite body and/or filler material whichis to be converted or altered can be varied. Thus, each of theabove-discussed processes could be limited to primarily a surface areaof a metal matrix composite body, or, if conversion was permitted tooccur for a sufficient amount of time, substantially complete conversionof the metal matrix composite body which was formed by spontaneousinfiltration (or any other appropriate technique) could occur. Moreover,factors such as temperature, time, atmospheric pressure, atmosphericcomposition, reacting species, particulate size of the reacting species,etc., could enhance or reduce the rate of conversion of at least aportion of the formed metal matrix composite body. Moreover, dependingon a desired result, metal matrix composite bodies may be modified usingany combination of the modification techniques discussed herein.

The objective of forming a heat treatable matrix metal in the formedmetal matrix composite body may also, in certain circumstances,influence the selection of a second material (or a precursor to a secondmaterial). For example, a second material could be added to a preform ora filler material, such second material providing (1) a second metal tothe matrix metal which enhances the ability of the matrix metal to beheat-treated and (2) desirable reaction product(s) due to a reaction of,for example, matrix metal with the second material.

In any of the above discussed alteration methods, it is possible toconduct such alterations in an atmosphere which is substantially similarto, or substantially different from, the atmosphere which is presentduring the formation of the composite bodies. To select an appropriateatmosphere, a number of different criteria should be examined includingwhether any desirable/undesirable reactions may occur between anyconstituent in the composite body and the atmosphere, whether theatmosphere favorably/unfavorably influences the formation of anydesirable/undesirable reaction products in the composite body, etc.Accordingly, depending on a totality of the circumstances, any one of anoxidizing atmosphere, a reducing atmosphere and/or an inert atmospheremay be used in combination with various preferred embodiments of theinvention.

One specific example of utilizing the preferred methods discussed aboveto modify the properties of a metal matrix composite is to increase theelectrical conductivity of the metallic constituent of a metal matrixcomposite body. Specifically, the presence of high quantities of siliconwithin an aluminum matrix metal alloy in metal matrix composite bodiesmay tend to reduce the electrical conductivity of the metal matrixcomposite body. By modifying the aluminum-silicon matrix metal of, forexample, a silicon carbide reinforced aluminum-silicon alloy matrixcomposite to include such metals as, for example, phosphorous, arsenicand/or antimony, the electrical conductivity of the silicon constituentin the matrix metal may be increased. Under appropriate reactionconditions, any of the preferred methods discussed above may be carriedout to modify the matrix metal.

In a still further embodiment, a gaseous means may be used to modify themetallic constituent and thus modify the properties of a formed metalmatrix composite. For example, a metal matrix composite may be contactedwith, for example, an oxidizing or nitriding or carburizing atmospherein order to modify (e.g., modify initially or post-treat afterinfiltrating) the composition of the matrix metal to achieve a desiredresult. Specifically, a gaseous medium containing an element whichreacts with at least a portion of a surface of the formed metal matrixcomposite body is flowed across the surface of a formed metal matrixcomposite body, thereby modifying the resultant properties of the formedbody.

In a further preferred embodiment of the present invention, the matrixmetal of a formed metal matrix composite may be modified by providing atleast one grain refiner within at least a portion of the filler materialor preform and/or matrix metal. Specifically, a grain refiner maycomprise any material (e.g., metal, oxide, nitride, carbide, etc.)which, under the processing conditions, initiates preferentialnucleation of at least one phase, other than the matrix metal phase,within the metallic constituent of the formed metal matrix composite,thus modifying the properties of the metal matrix composite. The grainrefiners are typically solid under the processing conditions. In aparticularly preferred embodiment, grain refiners may be created by, forexample, ball milling a filler material to break off, for example, edgesof the filler particulate. Under the processing conditions, these verysmall filler particles act as nucleation sites for the precipitation ofat least one phase, other than the matrix metal phase, within themetallic constituent. Moreover, rather than creating grain refiners byball milling the filler material, grain refiners may be added to atleast a portion of the filler material or preform and/or matrix metal inorder to achieve similar precipitation in the metallic constituent. Forexample, in the case of an aluminum matrix metal, suitable grainmodifiers may comprise alumina, titanium diboride, zirconium diboride,titanium aluminides, aluminum borides, manganese, and the like, andcombinations thereof.

To obtain spontaneous infiltration, an infiltration enhancer and/orinfiltration enhancer precursor and/or infiltrating atmosphere are incommunication with the filler material or preform, at least at somepoint during the process, which permits molten matrix metal tospontaneously infiltrate the filler material or preform. Specifically,in order to effect spontaneous infiltration of the matrix metal into thefiller material or preform, an infiltration enhancer should be providedto the spontaneous system. An infiltration enhancer could be formed froman infiltration enhancer precursor which could be provided (1) in thematrix metal; and/or (2) in the filler material or preform; and/or (3)from the infiltrating atmosphere and/or (4) from an external source intothe spontaneous system. Moreover, rather than supplying an infiltrationenhancer precursor, an infiltration enhancer may be supplied directly toat least one of the filler material or preform, and/or matrix metal,and/or infiltrating atmosphere. Ultimately, at least during thespontaneous infiltration, the infiltration enhancer should be located inat least a portion of the filler material or preform.

In a preferred embodiment it is possible that the infiltration enhancerprecursor can be at least partially reacted with the infiltratingatmosphere such that infiltration enhancer can be formed in at least aportion of the filler material or preform prior to or substantiallycontiguous with contacting the preform with molten matrix metal (e.g.,if magnesium was the infiltration enhancer precursor and nitrogen wasthe infiltrating atmosphere, the infiltration enhancer could bemagnesium nitride which would be located in at least a portion of thefiller material or preform).

An example of a matrix metal/infiltration enhancerprecursor/infiltrating atmosphere system is thealuminum/magnesium/nitrogen system. Specifically, an aluminum matrixmetal can be embedded within a filler material which can be containedwithin a suitable refractory vessel which, under the process conditions,does not react with the aluminum matrix metal and/or the filler materialwhen the aluminum is made molten. A filler material containing or beingexposed to magnesium, and being exposed to, at least at some pointduring the processing, a nitrogen atmosphere, can be contacted with themolten aluminum matrix metal. The matrix metal will then spontaneouslyinfiltrate the filler material or preform.

Under the conditions employed in the method of the present invention, inthe case of an aluminum/magnesium/nitrogen spontaneous system, thefiller material or preform should be sufficiently permeable to permitthe nitrogen-containing gas to penetrate or permeate the filler materialor preform at some point during the process and/or contact the moltenmatrix metal. Moreover, the permeable filler material or preform canaccommodate infiltration of the molten matrix metal, thereby causing thenitrogen-permeated filler material or preform to be infiltratedspontaneously with molten matrix metal to form a metal matrix compositebody and/or cause the nitrogen to react with an infiltration enhancerprecursor to form infiltration enhancer in the filler material orpreform, thereby resulting in spontaneous infiltration. The extent orrate of spontaneous infiltration and formation of the metal matrixcomposite will vary with a given set of process conditions, includingmagnesium content of the aluminum alloy, magnesium content of the fillermaterial or preform, amount of magnesium nitride in the filler materialor preform, the presence of additional alloying elements (e.g., silicon,iron, copper, manganese, chromium, zinc, and the like), average size ofthe filler material (e.g., particle diameter), surface condition andtype of filler material, nitrogen concentration of the infiltratingatmosphere, time permitted for infiltration and temperature at whichinfiltration occurs. For example, for infiltration of the moltenaluminum matrix metal to occur spontaneously, the aluminum can bealloyed with at least about 1% by weight, and preferably at least about3% by weight, magnesium (which functions as the infiltration enhancerprecursor), based on alloy weight. Auxiliary alloying elements, asdiscussed above, may also be included in the matrix metal to tailorspecific properties thereof. Additionally, the auxiliary alloyingelements may affect the minimum amount of magnesium required in thematrix aluminum metal to result in spontaneous infiltration of thefiller material or preform. Loss of magnesium from the spontaneoussystem due to, for example, volatilization should not occur to such anextent that no magnesium was present to form infiltration enhancer.Thus, it is desirable to utilize a sufficient amount of initial alloyingelements to assure that spontaneous infiltration will not be adverselyaffected by volatilization. Still further, the presence of magnesium inboth of the filler material and matrix metal or the filler materialalone may result in a reduction in the required amount of magnesium toachieve spontaneous infiltration (discussed in greater detail laterherein).

The volume percent of nitrogen in the nitrogen atmosphere also affectsformation rates of the metal matrix composite body. Specifically, ifless than about 10 volume percent of nitrogen is present in theatmosphere, very slow or little spontaneous infiltration will occur. Ithas been discovered that it is preferable for at least about 50 volumepercent of nitrogen to be present in the infiltrating atmosphere,thereby resulting in, for example, shorter infiltration times due to amuch more rapid rate of infiltration. The infiltrating atmosphere (e.g.,a nitrogen containing gas) can be supplied directly to the fillermaterial and/or matrix metal, or it may be produced or result from adecomposition of a material.

The minimum magnesium content required for molten matrix metal toinfiltrate a filler material or preform depends on one or more variablessuch as the processing temperature, time, the presence of auxiliaryalloying elements such as silicon or zinc, the nature of the fillermaterial, the location of the magnesium in one or more components of thespontaneous system, the nitrogen content of the atmosphere, and the rateat which the nitrogen atmosphere flows. Lower temperatures or shorterheating times can be used to obtain complete infiltration as themagnesium content of the alloy and/or filler material is increased.Also, for a given magnesium content, the addition of certain auxiliaryalloying elements such as zinc permits the use of lower temperatures.For example, a magnesium content of the matrix metal at the lower end ofthe operable range, e.g., from about 1 to 3 weight percent, may be usedin conjunction with at least one of the following: an above-minimumprocessing temperature, a high nitrogen concentration, or one or moreauxiliary alloying elements. When no magnesium is added to the fillermaterial, alloys containing from about 3 to 5 weight percent magnesiumare preferred on the basis of their general utility over a wide varietyof process conditions, with at least about 5 percent being preferredwhen lower temperatures and shorter times are employed. Magnesiumcontents in excess of about 10 percent by weight of the aluminum alloymay be employed to moderate the temperature conditions required forinfiltration. The magnesium content may be reduced when used inconjunction with an auxiliary alloying element, but these elements servean auxiliary function only and are used together with at least theabove-specified minimum amount of magnesium. For example, there wassubstantially no infiltration of nominally pure aluminum alloyed onlywith 10 percent silicon at 1000° C. into a bedding of 500 mesh, 39CRYSTOLON® (99 percent pure silicon carbide from Norton Co.). However,in the presence of magnesium, silicon has been found to promote theinfiltration process. As a further example, the amount of magnesiumvaries if it is supplied exclusively to the filler material. It has beendiscovered that spontaneous infiltration will occur with a lesser weightpercent of magnesium supplied to the spontaneous infiltration systemwhen at least some of the total amount of magnesium supplied is placedin the filler material. It may be desirable for a lesser amount ofmagnesium to be provided in order to prevent the formation ofundesirable intermetallics in the metal matrix composite body. In thecase of a silicon carbide preform, it has been discovered that when thepreform is contacted with an aluminum matrix metal, the preformcontaining at least about 1% by weight magnesium and being in thepresence of a substantially pure nitrogen atmosphere, the matrix metalspontaneously infiltrates the preform. In the case of an aluminapreform, the amount of magnesium required to achieve acceptablespontaneous infiltration is slightly higher. Specifically, it has beenfound that when an alumina preform, when contacted with a similaraluminum matrix metal, at about the same temperature as the aluminumthat infiltrated into the silicon carbide preform, and in the presenceof the same nitrogen atmosphere, at least about 3% by weight magnesiummay be required to achieve similar spontaneous infiltration to thatachieved in the silicon carbide preform discussed immediately above.

Further, when infiltrating a permeable filler (e.g., aluminum oxide)utilizing a aluminum/magnesium/nitrogen system a spinel (e.g., MgAl₂ O₄)can be formed. Thus, when a sufficient amount of magnesium is present,the magnesium can react with an alumina filler if held at a hightemperature for a sufficient period of time. The formation of MgAl₂ O₄results in the volumetric expansion and reduction of metal, as discussedabove.

It is also noted that it is possible to supply to the spontaneous systeminfiltration enhancer precursor and/or infiltration enhancer on asurface of the alloy and/or on a surface of the preform or fillermaterial and/or within the preform or filler material prior toinfiltrating the matrix metal into the filler material or preform (i.e.,it may not be necessary for the supplied infiltration enhancer orinfiltration enhancer precursor to be alloyed with the matrix metal, butrather, simply supplied to the spontaneous system). If the magnesium wasapplied to a surface of the matrix metal it may be preferred that saidsurface should be the surface which is closest to, or preferably incontact with, the permeable mass of filler material or vice versa; orsuch magnesium could be mixed into at least a portion of the fillermaterial. Still further, it is possible that some combination of surfaceapplication, alloying and placement of magnesium into at least a portionof the filler material could be used. Such combination of applyinginfiltration enhancer(s) and/or infiltration enhancer precursor(s) couldresult in a decrease in the total weight percent of magnesium needed topromote infiltration of the matrix aluminum metal into the fillermaterial, as well as achieving lower temperatures at which infiltrationcan occur. Moreover, the amount of undesirable intermetallics formed dueto the presence of magnesium could also be minimized.

The use of one or more auxiliary alloying elements and the concentrationof nitrogen in the surrounding gas also affects the extent of nitridingof the matrix metal at a given temperature. For example, auxiliaryalloying elements such as zinc or iron included in the alloy, or placedon a surface of the alloy, may be used to reduce the infiltrationtemperature and thereby decrease the amount of nitride formation,whereas increasing the concentration of nitrogen in the gas may be usedto promote nitride formation.

The concentration of magnesium in the alloy, and/or placed onto asurface of the alloy, and/or combined in the filler or preform material,also tends to affect the extent of infiltration at a given temperature.Consequently, in some cases where little or no magnesium is contacteddirectly with the preform or filler material, it may be preferred thatat least about three weight percent magnesium be included in the alloy.Alloy contents of less than this amount, such as one weight percentmagnesium, may require higher process temperatures or an auxiliaryalloying element for infiltration. The temperature required to effectthe spontaneous infiltration process of this invention may be lower: (1)when the magnesium content of the alloy alone is increased, e.g. to atleast about 5 weight percent; and/or (2) when alloying constituents aremixed with the permeable mass of filler material; and/or (3) whenanother element such as zinc or iron is present in the aluminum alloy.The temperature also may vary with different filler materials. Ingeneral, spontaneous and progressive infiltration will occur at aprocess temperature of at least about 675° C., and preferably a processtemperature of at least about 750° C.-800° C. Temperatures generally inexcess of 1200° C. do not appear to benefit the process, and aparticularly useful temperature range has been found to be from about675° C. to about 1200° C. However, as a general rule, the spontaneousinfiltration temperature is a temperature which is above the meltingpoint of the matrix metal but below the volatilization temperature ofthe matrix metal. Moreover, the spontaneous infiltration temperatureshould be below the melting point of the filler material. Still further,as temperature is increased, the tendency to form a reaction productbetween the matrix metal and infiltrating atmosphere increases (e.g., inthe case of aluminum matrix metal and a nitrogen infiltratingatmosphere, aluminum nitride may be formed). Such reaction product maybe desirable or undesirable, dependent upon the intended application ofthe metal matrix composite body. Additionally, electric resistanceheating is typically used to achieve the infiltrating temperatures.However, any heating means which can cause the matrix metal to becomemolten and does not adversely affect spontaneous infiltration isacceptable for use with the invention.

In the present method, for example, a permeable filler material orpreform comes into contact with molten aluminum in the presence of, atleast sometime during the process, a nitrogen-containing gas. Thenitrogen-containing gas may be supplied by maintaining a continuous flowof gas into contact with at least one of the filler material or thepreform and/or molten aluminum matrix metal. Although the flow rate ofthe nitrogen-containing gas is not critical, it is preferred that theflow rate be sufficient to compensate for any nitrogen lost from theatmosphere due to nitride formation in the alloy matrix, and also toprevent or inhibit the incursion of air which can have an oxidizingeffect on the molten metal.

The method of forming a metal matrix composite is applicable to a widevariety of filler materials, and the choice of filler materials willdepend on such factors as the matrix alloy, the process conditions, thereactivity of the molten matrix alloy with the filler material, theability of the filler material to conform to the matrix metal and theproperties sought for the final composite product. For example, whenaluminum is the matrix metal, suitable filler materials include (a)oxides, e.g. alumina; (b) carbides, e.g. silicon carbide; (c) borides,e.g. aluminum dodecarboride, and (d) nitrides, e.g. aluminum nitride. Ifthere is a tendency for the filler material to react with the moltenaluminum matrix metal, this might be accommodated by minimizing theinfiltration time and temperature or by providing a non-reactive coatingon the filler. The filler material may comprise a substrate, such ascarbon or other non-ceramic material, bearing an appropriate coating toprotect the substrate from attack or degradation. Suitable ceramiccoatings include oxides, carbides, borides and nitrides. Ceramics whichare preferred for use in the present method include alumina and siliconcarbide in the form of particles, platelets, whiskers and fibers. Thefibers can be discontinuous (in chopped form) or in the form of wovenmats or continuous filament, such as multifilament tows. Further, thefiller material may be homogeneous or heterogeneous.

Certain filler materials exhibit enhanced infiltration relative tofiller materials having a similar chemical composition. For example,crushed alumina bodies made by the method disclosed in U.S. Pat. No.4,713,360, entitled "Novel Ceramic Materials and Methods of MakingSame", which issued on Dec. 15, 1987, in the names of Marc S. Newkirk etal., exhibit desirable infiltration properties relative to commerciallyavailable alumina products. Moreover, crushed alumina bodies made by themethod disclosed in Commonly Owned U.S. Pat. No. 4,851,375, entitled"Methods of Making Composite Ceramic Articles Having Embedded Filler",which issued on Jul. 25, 1989, in the names of Marc S. Newkirk et al.,also exhibit desirable infiltration properties relative to commerciallyavailable alumina products. The subject matter of each of these issuedPatents is herein expressly incorporated by reference. Specifically, ithas been discovered that substantially complete infiltration of apermeable mass of a ceramic or ceramic composite material can occur atlower infiltration temperatures and/or lower infiltration times byutilizing a crushed or comminuted body produced by the method of theaforementioned U.S. Patents.

The size and shape of the filler material can be any that may berequired to achieve the properties desired in the composite. Thus, thefiller material may be in the form of particles, whiskers, platelets orfibers and mixtures thereof since infiltration is not restricted by theshape of the filler material. Other shapes such as spheres, tubules,pellets, refractory fiber cloth, and the like may be employed. Inaddition, the size of the material does not limit infiltration, althougha higher temperature or longer time period may be needed for completeinfiltration of a mass of smaller particles than for larger particles.Further, the mass of filler material (shaped into a preform) to beinfiltrated should be permeable (i.e., permeable to molten matrix metaland to the infiltrating atmosphere).

The method of forming metal matrix composites according to the presentinvention, not being dependent on the use of pressure to force orsqueeze molten matrix metal into a mass of filler material, permits theproduction of substantially uniform metal matrix composites having ahigh volume fraction of filler material and low porosity. Higher volumefractions of filler material on the order of at least about 50% may beachieved by using a lower porosity initial mass of filler material andparticles of varying size. Higher volume fractions also may be achievedif the mass of filler is compacted or otherwise densified provided thatthe mass is not converted into either a compact with close cell porosityor into a fully dense structure that would prevent infiltration by themolten alloy.

It has been observed that for aluminum infiltration and matrix formationaround a ceramic filler, wetting of the ceramic filler by the aluminummatrix metal may be an important part of the infiltration mechanism.Moreover, at low processing temperatures, a negligible or minimal amountof metal nitriding occurs resulting in a minimal discontinuous phase ofaluminum nitride dispersed in the metal matrix. However, as the upperend of the temperature range is approached, nitridation of the metal ismore likely to occur. Thus, the amount of the nitride phase in the metalmatrix can be controlled by varying the processing temperature at whichinfiltration occurs. The specific process temperature at which nitrideformation becomes more pronounced also varies with such factors as thematrix aluminum alloy used and its quantity relative to the volume offiller, the filler material to be infiltrated, and the nitrogenconcentration of the infiltrating atmosphere. For example, the extent ofaluminum nitride formation at a given process temperature is believed toincrease as the ability of the alloy to wet the filler decreases and asthe nitrogen concentration of the atmosphere increases.

It is therefore possible to tailor the constituency of the metal matrixduring formation of the composite to impart certain characteristics tothe resulting product. For a given system, the process conditions can beselected to control the nitride formation. A composite productcontaining an aluminum nitride phase will exhibit certain propertieswhich can be favorable to, or improve the performance of, the product.Further, the temperature range for spontaneous infiltration with analuminum alloy may vary with the ceramic material used. In the case ofalumina as the filler material, the temperature for infiltration shouldpreferably not exceed about 1000° C. if it is desired that the ductilityof the matrix not be reduced by the significant formation of nitride.However, temperatures exceeding 1000° C. may be employed if it isdesired to produce a composite with a less ductile and stiffer matrix.To infiltrate silicon carbide, higher temperatures of about 1200° C. maybe employed since the aluminum alloy nitrides to a lesser extent,relative to the use of alumina as filler, when silicon carbide isemployed as a filler material.

Moreover, it is possible to use a reservoir of matrix metal to assurecomplete infiltration of the filler material and/or to supply a secondmetal which has a different composition from the first source of matrixmetal. Specifically, in some cases it may be desirable to utilize amatrix metal in the reservoir which differs in composition from thefirst source of matrix metal. For example, if an aluminum alloy is usedas the first source of matrix metal, then virtually any other metal ormetal alloy which was molten at the processing temperature could be usedas the reservoir metal. Molten metals frequently are very miscible witheach other which would result in the reservoir metal mixing with thefirst source of matrix metal, so long as an adequate amount of time isgiven for the mixing to occur. Thus, by using a reservoir metal which isdifferent in composition than the first source of matrix metal, it ispossible to tailor the properties of the metal matrix to meet variousoperating requirements and thus tailor the properties of the metalmatrix composite.

A barrier means may also be utilized in combination with the presentinvention. Specifically, the barrier means for use with this inventionmay be any suitable means which interferes, inhibits, prevents orterminates the migration, movement, or the like, of molten matrix alloy(e.g., an aluminum alloy) beyond the defined surface boundary of thefiller material. Suitable barrier means may be any material, compound,element, composition, or the like, which, under the process conditionsof this invention, maintains some integrity, is not volatile andpreferably is permeable to the gas used with the process as well asbeing capable of locally inhibiting, stopping, interfering with,preventing, or the like, continued infiltration or any other kind ofmovement beyond the defined surface boundary of the filler material.

Suitable barrier means includes materials which are substantiallynon-wettable by the migrating molten matrix alloy under the processconditions employed. A barrier of this type appears to exhibit little orno affinity for the molten matrix alloy, and movement beyond the definedsurface boundary of the filler material is prevented or inhibited by thebarrier means. The barrier reduces any final machining or grinding thatmay be required of the metal matrix composite product. As stated above,the barrier preferably should be permeable or porous, or renderedpermeable by puncturing, to permit the gas to contact the molten matrixalloy.

Suitable barriers particularly useful for aluminum matrix alloys arethose containing carbon, especially the crystalline allotropic form ofcarbon known as graphite. Graphite is essentially non-wettable by themolten aluminum alloy under the described process conditions. Aparticularly preferred graphite is a graphite tape product that is soldunder the trademark Grafoil®, registered to Union Carbide. This graphitetape exhibits sealing characteristics that prevent the migration ofmolten aluminum alloy beyond the defined surface boundary of the fillermaterial. This graphite tape is also resistant to heat and is chemicallyinert. Grafoil® graphite material is flexible, compatible, conformableand resilient. It can be made into a variety of shapes to fit anybarrier application. However, graphite barrier means may be employed asa slurry or paste or even as a paint film around and on the boundary ofthe filler material. Grafoil® is particularly preferred because it is inthe form of a flexible graphite sheet. In use, this paper-like graphiteis simply formed around the filler material.

Other preferred barrier(s) for aluminum metal matrix alloys in nitrogenare the transition metal borides (e.g., titanium diboride (TiB₂)) whichare generally non-wettable by the molten aluminum metal alloy undercertain of the process conditions employed using this material. With abarrier of this type, the process temperature should not exceed about875° C., for otherwise the barrier material becomes less efficaciousand, in fact, with increased temperature infiltration into the barrierwill occur. The transition metal borides are typically in a particulateform (1-30 microns). The barrier materials may be applied as a slurry orpaste to the boundaries of the permeable mass of ceramic filler materialwhich preferably is preshaped as a preform.

Other useful barriers for aluminum metal matrix alloys in nitrogeninclude low-volatile organic compounds applied as a film or layer ontothe external surface of the filler material. Upon firing in nitrogen,especially at the process conditions of this invention, the organiccompound decomposes leaving a carbon soot film. The organic compound maybe applied by conventional means such as painting, spraying, dipping,etc.

Moreover, finely ground particulate materials can function as a barrierso long as infiltration of the particulate material would occur at arate which is slower than the rate of infiltration of the fillermaterial.

Thus, the barrier means may be applied by any suitable means, such as bylayering the defined surface boundary with the barrier means. Such layerof barrier means may be applied by painting, dipping, silk screening,evaporating, or otherwise applying the barrier means in liquid, slurry,or paste form, or by sputtering a vaporizable barrier means, or bysimply depositing a layer of a solid particulate barrier means, or byapplying a solid thin sheet or film of barrier means onto the definedsurface boundary. With the barrier means in place, spontaneousinfiltration substantially terminates upon reaching the defined surfaceboundary and contacting the barrier means.

The following Examples are intended to further illustrate the invention.

EXAMPLE 1

A filler material mixture was prepared by combining in a plastic jar amixture of, by weight, about 54.3% 39 CRYSTOLON® 54 grit silicon carbide(Norton Co., Worcester, Mass.), about 23.3% 39 CRYSTOLON® 90 gritsilicon carbide, about 31% 39 CRYSTOLON® 1000 grit silicon carbide, andabout 19.3% BLUONIC® A colloidal alumina (Bantrock Industries, Inc.,Lively, Va.).

The filler material mixture was roll mixed for about 45 minutes, thenpoured into a silicone rubber mold with an internal cavity measuringabout 7 inches (178 mm) square and about 1.5 inches (38 mm) deep. Therubber mold was vibrated to assist in sedimentation. After vibrating forabout 0.5 hour, the excess water on the surface of the formed sedimentcast preform was removed with a paper towel. The silicone moldcontaining the preform was vibrated for approximately an additionalhour, then the remaining surface water was removed and the siliconerubber mold was transferred from the vibration table into a freezer.After the residual water in the preform had thoroughly frozen, thesilicone rubber mold and its preform were removed from the freezer, andthe frozen sediment cast preform was withdrawn from the rubber mold. Thepreform was placed on a bed of 38 ALUNDUM® 90 grit alumina particulatematerial and allowed to dry in air at room temperature for about 16hours.

Referring to FIG. 1, after drying, the sediment cast preform 13,measuring about 7 inches (178 mm) square and about 1.38 inches (35 mm)thick, was transferred to a new bedding of 90 grit alumina supported bya refractory plate and placed into a resistance heated air atmospherefurnace for firing. The furnace temperature was increased from aboutroom temperature to about 1050° C. in a period of about 10 hours. Afterabout 2 hours at about 1050° C., the furnace and its contents werecooled to about room temperature in about 10 hours.

A GRAFOIL® graphite foil (Union Carbide Company, Carbon ProductsDivision, Cleveland, Ohio) box 12 measuring about 8.5 inches (216 mm)square and about 4 inches (102 mm) high was placed into a graphite boat11 measuring about 9 inches (229 mm) square and about 4 inches (102 mm)high in its interior. The fired sediment cast preform 13 was then placedinto the bottom of the graphite foil box 12. A bedding material 14comprising by weight about 15% borosilicate glass frit (F-69 FusionCeramics, Inc., Carrollton, Ohio), about 28.3% E1 ALUNDUM® 90 gritalumina (Norton Co., Worcester, Mass.), about 28.3% E1 ALUNDUM® 220 gritalumina, and about 28.4% E1 ALUNDUM® 500 grit alumina, was placed intothe graphite foil box 12 around the fired sediment cast preform 13 to alevel substantially flush with the top of the preform. A thin surfacelayer 15 of -100 mesh magnesium powder (Hart Corp., Tamaqua, Pa.) wassprinkled over the top of the preform 13.

A gating means comprising a graphite ring 17, having an inner diametermeasuring about 2.5 inches (64 mm) and a height of about 0.5 inch (13mm), was centered over an about 2.5 inch (64 mm) diameter hole in anapproximately 7 inch (178 mm) square by 14 mil (0.36 mm) thick sheet ofgraphite foil 16. The graphite ring 17 was then adhered to the graphitefoil 16 with a thin layer of an adhesive comprising about 40% volumepercent RIGIDLOCK® graphite cement (Polycarbon Corp., Valencia, Calif.)and the balance ethanol. The joined graphite ring and the graphite foilwere allowed to dry in air at room temperature for about 4 hours.

The graphite foil and graphite foil ring assembly were then placed ontothe layer of -100 mesh magnesium powder 15 in the graphite foil box 12with the graphite ring 17 facing up. The inside of the graphite ring 17was then filled with a particulate mixture 19 comprising by weight about1% -100 mesh magnesium powder, about 1% -325 mesh magnesium powder,about 29% 39 CRYSTOLON® 90 grit silicon carbide, and the balance 39CRYSTOLON® 54 grit green silicon carbide. Additional bedding material 14(particulate mixture of alumina and borosilicate glass frit) was thenpoured into the graphite box 12 on the graphite foil 16 around thegraphite ring 17 to a height substantially flush with the top of thegraphite ring 17, and somewhat higher out towards the walls of thegraphite foil box 12. An ingot of matrix metal 18 weighing about 2965grams and comprising by weight about 15% Si, 5% Mg, and the balance ofaluminum, was placed into the graphite foil box 12 and centered over thegraphite ring 17 to complete the lay-up 10.

The graphite boat 11 and its contents were placed into a resistanceheated controlled atmosphere furnace at room temperature. The furnacechamber was evacuated to a vacuum of about 30 inches (762 mm) of mercuryand then backfilled with nitrogen gas to establish a gas flow rate ofabout 4 liters per minute. The furnace temperature was increased toabout 200° C. at a rate of about 150° C. per hour. After about 44 hours,at about 200° C., the furnace and its contents were heated to about 825°C. at a rate of about 150° C. per hour while maintaining a nitrogen flowrate of about 4 liters per minute. After maintaining a temperature ofabout 825° C. for about 18 hours, the temperature was then decreased toabout 675° C. at a rate of about 200° C. per hour. At a temperature ofabout 675° C., the graphite boat and its contents were removed from thefurnace and placed onto a water cooled aluminum quench plate. A FEEDOL®9 particulate hot topping material (Foseco, Inc., Cleveland, Ohio) waspoured onto the top of the residual molten matrix metal. Anapproximately 2 inch (51 mm) thick layer of CERABLANKET® ceramic fiberinsulation (Manville Refractory Products, Denver, Colo.) was placed ontop of the lay-up, and the graphite boat to further assist indirectional solidification. After cooling to substantially roomtemperature, the lay-up was removed from the graphite boat. The beddingof alumina and glass frit material was removed from around the lay-upwith lighter hammer blows to reveal that the matrix metal hadinfiltrated the sediment cast preform to produce a metal matrixcomposite body.

The metal matrix composite body was sectioned, mounted, polished, andexamined with an optical microscope. It was observed that the matrixmetal of the composite contained substantially no Mg₂ Si precipitates.Closer examination of the microstructure of the metal matrix compositeusing an electron microscope and energy-dispersion analysis at about1500× suggested that the alumina introduced into the preform ascolloidal alumina had at least partially reacted with magnesium of thematrix metal to form mixed oxides of aluminum and magnesium.

EXAMPLE 2

The following Example demonstrates a method for modifying the matrixmetal within a metal matrix composite body. Specifically, a formed metalmatrix composite body was placed into a powder bedding of a secondmetal, was heated to a temperature below the melting temperature of thematrix metal and was held at the elevated temperature for a timesufficient to allow the matrix metal and the second metal tointerdiffuse, thereby altering the chemical composition of the metallicconstituent of the resultant metal matrix composite body.

Specifically, FIG. 2 shows a schematic cross-sectional view of the setupused to produce a metal matrix composite sample, as described below. Asilica mold 40 was prepared, having an inner cavity measuring about 5inches (127 mm) long by about 5 inches (127 mm) wide by about 3.25inches (83 mm) deep, and having five holes 41 in the bottom of the mold,each measuring about 0.75 inches (19 mm) in diameter and about 0.75inches (19 mm) deep. The mold was formed by first mixing a slurrycomprising by weight of about 2.5 parts RANCO-SIL™4 -200 mesh silicapowder (Ransom & Randolph, Maunee, Ohio), about 1 part NYACOL® 830colloidal silica (Nyacol Products, Inc., Ashland, Mass.) and about 1.5parts RANCO-SIL™ A -50 mesh, +100 mesh silica sand (Ransom & Randolph,Maunee, Ohio). The slurry mixture was poured into a rubber mold having anegative shape of the desired inner cavity of the silica mold and placedin a freezer overnight (about 14 hours). The silica mold 40 wassubsequently separated from the rubber mold, fired at about 800° C. inan air atmosphere furnace for about 1 hour and cooled to roomtemperature.

The bottom surface of the formed silica mold 40 was covered with a pieceof PERMA FOIL® graphite foil 42 (TTAmerica, Portland, Oreg.), havingdimensions of about 5 inches (127 mm) long by about 5 inches (127 mm)wide by about 0.010 inches (0.25 mm) thick. Holes 43, about 0.75 inches(19 mm) in diameter, were cut into the graphite foil to correspond inposition to the holes 41 in the bottom of the silica mold 40. The holes41 in the bottom of the silica mold 40 were filled with matrix metalcylinders 44, measuring about 0.75 inches (19 mm) in diameter by about0.75 inches (19 mm) thick, having a composition identical to the matrixmetal, as described below. About 826 grams of a filler material mixture45, comprising by weight about 95 percent 38 ALUNDUM® 220 grit alumina(Norton, Co., Worcester, Mass.) and about 5 percent AESAR® -325magnesium powder (Al Corp./AESAR® Johnson Matthey, Seabrook, N.H.), wereprepared in an about 4 liter plastic jar by hand shaking for about 15minutes. The filler material mixture 45 was then poured into the bottomof the silica mold 40 to a depth of about 0.75 inch (19 mm) and tappedlightly to level the surface of the filler material mixture. About 1220grams of a matrix metal 46, comprising by weight approximately ≦0.25%Si, ≦0.30% Fe, ≦0.25% Cu, ≦0.15% Mn, about 9.5-10.6% Mg, ≦0.15% Zn,≦0.25% Ti and the balance aluminum, were placed on top of the fillermaterial mixture 45 within the silica mold 40. The silica mold 40 andits contents were then placed into a stainless steel container 47,having dimensions of about 10 inches (254 mm) long by about 10 inches(254 mm) wide by about 8 inches (203 mm) high. A titanium spongematerial 48, weighing about 15 grams (from Chemalloy Inc., Bryn Mawr,Pa.), was sprinkled around the silica mold 40 in the stainless steelcontainer 47. A sheet of copper foil 49 was placed over the opening ofthe stainless steel container 47, so as to form an isolated chamber. Anitrogen purge tube 50 was provided through the sheet of copper foil 49,and the stainless steel container 47 and its contents were placed intoan air atmosphere resistance heated box furnace.

The furnace was ramped from about room temperature to about 600° C. at arate of about 400° C./hour with a nitrogen flow rate of about 10liters/minute (note that the isolated chamber may not be completely gastight and permit, typically, some nitrogen to escape therefrom), thenheated from about 600° C. to about 750° C. at a rate of about 400°C./hour with a nitrogen flow rate of about 2 liters/minute. Afterholding the system at about 775° C. for approximately 1.5 hours with anitrogen flow rate of about 2 liters/minute, the stainless steelcontainer 47 and its contents were removed from the furnace. The silicamold 40 was removed from the stainless steel container 47, and a portionof the residual matrix metal was decanted from within the silica mold40. A room temperature copper chill plate, about 5 inches (127 mm) longby about 5 inches (127 mm) wide by about 1 inch (25 mm) thick, wasplaced within the silica mold 40 such that it contacted the top portionof the residual matrix metal, to directionally solidify the formed metalmatrix composite. At about room temperature, the metal matrix compositewas cut with a diamond saw into pieces measuring about 0.63 inch (16 mm)long by about 0.5 inch (13 mm) wide by and about 0.35 inch (9 mm) thick.

The formed metal matrix composite body was then subjected to a modifyingtreatment. Specifically, as shown in FIG. 3, an approximately 0.5 inch(13 mm) layer of -100 mesh copper powder 35 (Consolidated Astronautics,Inc., Saddle Brook, N.J.) was placed into a container 31 made from AISDIType 304 stainless steel, measuring about 2.25 inches (57 mm) high andhaving an outer diameter of about 1.5 inches (38 mm) and an innerdiameter of about 1.38 inches (35 mm). Prior to placing the copperpowder into the mold, a GRAFOIL® graphite foil lining 32 was placed intothe stainless steel container 31. A cut piece of the metal matrixcomposite 33 was placed onto the copper powder 35 in the graphite foillined stainless steel container. Additional -100 mesh copper powder 35was placed into the container to surround and cover the metal matrixcomposite to a level of about 0.5 inch (13 mm) above the top of thecomposite 33. A second metal matrix composite body 34 was placed in thecontainer and, again, the composite body covered with -100 mesh copperpowder 35 to a level of about 0.5 inch (13 mm) above the composite, thuscompleting the setup 36.

The setup 36 and its contents were placed into a controlled atmospherefurnace at about room temperature, and a flowing argon atmosphere ofabout 0.5 liters per minute was established with the furnace. Thefurnace and its contents were heated to about 600° C. in about 3 hoursand held at about 600° C. for about 2 hours, then cooled to about roomtemperature in about 3 hours.

At about room temperature, the setup was disassembled and the treatedsample was sectioned, mounted, and polished. Examination of themicrostructure with an optical microscope revealed that intermetallicshad formed in the matrix metal. Additional examination with an electronmicroscope using energy-dispersive x-ray analysis (EDAX) indicated thatthe intermetallic species comprised copper, magnesium, and alumina.Specifically, FIG. 4 is an secondary electron image of themicrostructure taken at about 1000× showing the alumina reinforcement52, the copper containing intermetallics 53 and the matrix metal 54.

EXAMPLE 3

The following Example demonstrates a method for modifying the morphologyof the precipitates which form in the matrix metal of a metal matrixcomposite body by ball milling the filler material mixture prior toforming the metal matrix composite body by a spontaneous infiltrationtechnique. Specifically, this Example demonstrates that theincorporation of fine particles resulting from the breaking of smallportions from the filler material mixture during the ball millingoperation introduces nucleation sites within the matrix metal thatmodifies the size and/or morphology of the resultant precipitates in thematrix metal. Table I summarizes the filler material, the matrix metal,the ball milling time, the ultimate tensile strength (UTS), the elasticmodulus, the coefficient of thermal expansion, (CTE) the fracturetoughness, and the density of the metal matrix composite bodies formedin this Example, as described below.

                                      TABLE I                                     __________________________________________________________________________                  Hours                          Fracture                             Filler                                                                             Matrix                                                                             Ball       Strain to                                                                            Elastic                                                                            CTE (°C..sup.-1                                                                Toughness                                                                            Density                   Sample                                                                            Material                                                                           Metal                                                                              Milled                                                                            UTS (MPa)                                                                            Failure (%)                                                                          Mod. (E)                                                                           10.sup.-6)                                                                            (MPA-m.sup.1/2)                                                                      (g/cm.sup.3)              __________________________________________________________________________    A   220#SiC.sup.1                                                                      Al-12Si-2                                                                          24  228    0.212  188  12.8    12.79  2.77                               Mg                                                                   B   220#SiC.sup.1                                                                      Al-12Si-2                                                                          12  209    0.219  170  12.1    11.86  2.87                               Mg                                                                   C   220#SiC.sup.1                                                                      Al-12Si-2                                                                           6  209    0.211  172  12.8    11.15  2.80                               Mg                                                                   D   220#SiC.sup.1                                                                      Al-12Si-2                                                                           0  145    0.133  164  12.2    10.37  2.87                               Mg                                                                   __________________________________________________________________________     .sup.1 39 CRYSTOLON ® silicon carbide (Norton Company, Worcester, MA)

Sample A

A filler material mixture was made by placing about 10,000 grams ofBURUNDUM® alumina milling media (U.S. Stoneware, Mahwah, N.J.),measuring about 15/16 inch (24 mm) in diameter, into an 8.9 literporcelain ball mill (U.S. Stoneware Corp.). About 5000 grams of 39CRYSTOLON® 220 grit silicon carbide (Norton Company, Worcester, Mass.)were added to the mill and the mixture was dry ball milled for about 24hours. The milling media was then removed and about 100 grams of -325mesh magnesium powder (AESAR®, Johnson Matthey Company, Seabrook, N.H.)were added. The admixture of ball milled silicon carbide and magnesiumparticulate was then roll mixed for about 2 hours.

A graphite foil box measuring about 8 inches (203 mm) long by about 4inches (102 mm) wide by about 5 inches (127 mm) high was made from apiece of GRAFOIL® graphite foil (Union Carbide Company, Danbury, Conn.)measuring about 14 inches (356 mm) long by about 12.5 inches (318 mm)wide by about 15 mils (0.38 mm) thick. Four parallel cuts, about 5inches (127 mm) from the side and about 5 inches (127 mm) long, weremade into the graphite foil. The cut graphite foil was then folded intoa graphite foil box with edges glued together with RIGIDLOCK® graphitecement (Polycarbon Corporation, Valencia, Calif.). The bottom of theinterior of the graphite foil box was uniformly coated with a mixturecomprising by volume equal parts of RIGIDLOCK® graphite cement andPHARMCO™ ethyl alcohol (Pharmco Products, Inc., Bayonne, N.J.). The wetgraphite cement layer on the bottom of the inside of the graphite foilbox was then uniformly coated with a layer of -50 mesh magnesiumparticulate (ALFA® Products, Morton Thiokol Inc., Danvers, Mass.).

A quantity of the roll mixed filler admixture was poured into thegraphite foil box to a depth of about 1 inch (25 mm). After leveling thefiller material admixture, about 7 grams of AESAR® magnesium particulate(-50 mesh) was sprinkled evenly over the surface of the leveled fillermaterial. A matrix metal ingot measuring about 4 inches (102 mm) wide byabout 41/2 inches (114 mm) long by about 11/2 inches (38 mm) thick,weighing about 1200 grams and comprising by weight about 12 percentsilicon, 2 percent magnesium and the balance aluminum, was placed intothe graphite foil box and oriented such that one of the about 41/2 by11/2 inch (114 by 38 mm) faces contacted the magnesium particulate layerin the box.

The graphite boat and its contents were then placed into a resistanceheated controlled atmosphere furnace. The furnace chamber was evacuatedto a vacuum of about 30 (762 mm) inches of mercury and then backfilledwith nitrogen gas to establish a nitrogen gas flow rate of about 2.5liters per minute. The furnace temperature was increased from about roomtemperature to about 750° C. at a rate of about 150° C. per hour. Aftermaintaining a temperature of about 750° C. for about 15 hours, thetemperature was decreased to about 675° C. at a rate of about 150° C.per hour. At a temperature of about 675° C., the graphite boat and itscontents were removed from the furnace and placed onto a roomtemperature graphite chill plate to directionally solidify the formedmetal matrix composite and the residual matrix metal reservoir. Afterthe residual matrix metal had cooled to substantially room temperature,the graphite foil box and its contents were removed from the graphiteboat and the graphite foil was peeled away to reveal that matrix metalhad infiltrated the filler material admixture to form a metal matrixcomposite.

Samples B and C

Metal matrix composite Samples B and C were formed in substantially thesame manner as was metal matrix composite Sample A, except that the 220grit silicon carbide filler was ball milled for 12 hours for Sample Band for 6 hours for Sample C.

Sample D

Metal matrix composite Sample D was formed in substantially the samemanner as metal matrix composite Samples A, B and C, except that the 220grit silicon carbide filler material was not ball milled (however, thetwo hour roll mixing operation of the silicon carbide and magnesiumparticulates was retained).

The metal matrix composite bodies of this Example were cut, mounted, andpolished to compare the microstructures of the resultant bodies.Specifically, FIGS. 5a, 5b, 5c, and 5d are photomicrographs taken atabout 400× magnification which correspond to Samples A, B, C and D,respectively. These micrographs show that, as the ball milling time isincreased, the amount of fine particles in the metal matrix bodyincreases. The micrographs also show that as the amount of fineparticles is increased, the quantity and morphology of precipitates inthe metal matrix composite body is altered.

Properties of the formed metal matrix composite bodies in this Exampleare set forth in Table 1. Examination of these tabulated propertiesshows that ball milling the filler material significantly increases thestrength and toughness of the subsequently formed metal matrixcomposite.

Details of the test procedures for measuring the properties of the metalmatrix composite bodies are set forth below.

Measurement of Ultimate Tensile Strength (U.T.S.)

The tensile strength of some metal matrix composites was determinedusing ASTM #B557-84 "Standard Methods of Tension Testing Wrought andCast Aluminum and Magnesium Products". Rectangular tension testspecimens having dimensions of 6 inches (154 mm) long by 0.5 inch (13mm) wide and 0.1 inches (2.5 mm) thick were used. The gauge section ofthe rectangular tensile test specimens was about 3/8 inch (10 mm) wideby about 0.75 inches (19 mm) long and the radii from end section to thegauge section were about 3 inches (76 mm). Four aluminum gripping tabs,about 2 inches (51 mm) long by about 0.5 inch (13 mm) wide and about 0.3inches (7.6 mm) thick, were fastened to the end sections of eachrectangular tension test specimens with an epoxy (designatedEpoxy-patch™, Dexter Corporation of High Sol Aerospace and IndustrialProducts, Seabrook, N.H.). The strain of the rectangular tension testspecimens was measured with strain gauges (350 ohm bridges) designatedCEA-06-375UW-350 from Micromeasurements of Raleigh, N.C. The rectangulartension test specimens, including the aluminum gripping tabs and straingauges, were placed in wedge grips on a Syntec 5000 pound (2269 kg) loadcell (Universal Testing Machine, Model No. CITS 2000/6 manufactured bySystem Integration Technology Inc. of Straton, Mass.). A computer dataacquisition system was connected to the measuring unit, and the straingauges recorded the test responses. The rectangular tension testspecimens were deformed at a constant rate of 0.039 inches/minute (1mm/minute) to failure. The ultimate tensile stress, maximum strain andstrain to failure were calculated from the sample geometry and recordedresponses using the relevant formulas internally programmed within thecomputer.

Measurement of Modulus by the Resonance Method

The elastic modulus of the metal matrix composites was determined by asonic resonance technique which is substantially the same as ASTM methodC848 -88. Specifically, a composite sample measuring from about 1.8 to2.2 inches long, about 0.24 inches wide and about 1.9 inches thick(about 45 mm to about 55 mm long, about 6 mm wide and about 4.8 mmthick) was placed between two transducers isolated from room vibrationsby an air table supporting a granite stone. One of the transducers wasused to excite frequencies within the composite sample while the otherwas used to monitor the frequency response of the metal matrixcomposite. By scanning through frequencies, monitoring and recording theresponse levels for each frequency, and noting the resonant frequencythe elastic modulus was determined.

Measurement of the Fracture Toughness for Metal Matrix Material Using aChevron Notch Specimen

The method of Munz, Shannon and Bubsey, was used to determine thefracture toughness of metal matrix materials. The fracture toughness wascalculated from the maximum load of Chevron notch specimen in four pointloading. Specifically, the geometry of the Chevron notch specimen wasabout 1.8 to 2.2 inches (45 to 55 mm) long, about 0.19 inches (4.8 mm)wide and about 0.24 inches (6 mm) high. A Chevron notch was cut with adiamond saw to propagate a crack through the sample. The Chevron notchedsamples, the apex of the Chevron pointing down, were placed into afixture within a Universal test machine. The notch of the Chevron notchsample, was placed between two pins 1.6 inches (40 mm) apart andapproximately 0.79 inch (20 mm) from each pin. The top side of theChevron notch sample was contacted by two pins 0.79 inch (20 mm) apartand approximately 0.39 inch (10 mm) from the notch. The maximum loadmeasurements were made with a Sintec Model CITS-2000/6 Universal TestingMachine manufactured by System Integration Technology Incorporated ofStraton, Mass. A cross-head speed of 0.02 inches/minute (0.58millimeters/minute) was used. The load cell of the Universal testingmachine was interfaced to a computer data acquisition system. Chevronnotch sample geometry and maximum load were used to calculate thefracture toughness of the material. Several samples were used todetermine an average fracture toughness for a given material.

EXAMPLE 4

This Example demonstrates that the filler reinforcement loading in ametal matrix composite can be reduced, while simultaneously modifyingthe composition of the matrix metal of the composite, by adding to thefiller material or preform a powdered metal or metal alloy having acomposition different from the matrix metal.

A graphite foil box measuring about 31/4 inches (83 mm) long by about13/4 inches (44 mm) wide by about 41/2 inches (114 mm) high wasfabricated from a single sheet of GRAFOIL® graphite foil (Union CarbideCompany, Danbury, Conn.), measuring about 0.015 inch (0.38 mm) thick, bymaking strategically placed cuts and folds in the sheet. The foldedportions of the GRAFOIL® box were cemented together with RIGIDLOCK®graphite cement (Polycarbon Corporation, Valencia, Calif.), and thecemented portions were further reinforced by stapling the box. Theformed GRAFOIL® box was then placed into a graphite boat having a wallthickness of about 1/2 inch (13 mm) and measuring about 9 inches (229mm) by about 5 inches (127 mm) by about 4 inches (102 mm) high.

About 200 grams of a filler material admixture comprising by weightabout 20 percent copper particulate, about 1.6% -325 mesh magnesiumparticulate (Reade Manufacturing Company, Lakehurst, N.J.) and thebalance Grade T-64 -325 mesh tabular alumina (Alcoa Industrial ChemicalsDivision, Bauxite, Ark.) were placed into a dry, approximately 1.1 literporcelain ball mill (U.S. Stoneware Corporation, Mahwah, N.J.)containing about 400 grams of approximately 1/2 inch (13 mm) diameterBURUNDUM® ball milling media (U.S. Stoneware, Mahwah, N.J.). The ballmill lid was secured, and the filler material admixture was ball milledfor about 2 hours. After ball milling, about 99 grams of the fillermaterial admixture was poured into the GRAFOIL® box and leveled. About1/2 gram of -100 mesh magnesium particulate (Hart Corporation, Tamaqua,Pa.) was sprinkled evenly over the surface of the filler materialadmixture. An approximately 446 gram ingot of matrix metal measuringabout 15/8 inches (41 mm) square and about 4 inches (102 mm) high andcomprising commercially pure aluminum (Aluminum Association 170.1), wassandblasted to remove any surface oxide present thereon. The ingot wasthen cleaned with ethyl alcohol to remove any debris from thesandblasting operation and placed onto the layer of magnesiumparticulate in the GRAFOIL® box.

The graphite boat and its contents were placed into a retort within acontrolled atmosphere furnace at substantially room temperature. Theretort was sealed, evacuated to a vacuum about 30 inches (762 mm) ofmercury and then backfilled with nitrogen gas to establish a nitrogengas flow rate of about 5 liters per minute. The temperature in thefurnace was then increased to about 800° C. at a rate of about 200° C.per hour, maintained at about 800° C. for about 15 hours, then decreasedto about 760° C. at a rate of about 200° C. per hour. At a temperatureof about 760° C., the graphite boat and its contents were removed fromthe retort and placed onto a water cooled aluminum quench plate. FEEDOL®No. 9 hot topping particulate mixture (Foseco, Inc., Cleveland, Ohio)was then poured over the residual molten matrix metal. After theexothermic reaction from the hot topping mix had substantially subsided,the top and sides of the graphite boat were covered with anapproximately 2 inch (51 mm) thick layer of CERABLANKET® ceramic fiberblanket material (Manville Refractory Products, Denver, Colo.). At aboutroom temperature, the lay-up was removed from the graphite boat and theGRAFOIL® box was disassembled to reveal that matrix metal hadinfiltrated the filler material admixture to form a metal matrixcomposite body.

Quantitative image analysis of the formed metal matrix composite bodywas carried out using a Nikon microphoto-FX optical microscope with aDAGE-MTI series 68 video camera (DAGE-MTI Inc., Michigan City, Ind.)attached to the top port. The video camera signal was sent to a ModelDV-4400 scientific optical analysis system (Lamont Scientific, StateCollege, Pa.). Results of the quantative image analysis revealed thatthe volume fraction of the tabular alumina constituent of the fillermaterial was approximately 47 volume percent.

Semi-quantitative analysis was performed on the matrix metal phase ofthe metal matrix composite body to determine the constituents presentwithin the matrix metal. Analysis was carried out using the energydispersive X-ray analysis (EDAX) feature (Model VZ15, Princeton GammaTech, Inc., Princeton, N.J.) on a scanning electron microscope (Model500, Philips NV, Eindhoven, The Netherlands) coupled to a spectrumanalyzer (Tracor Northern Inc., Middleton, Wis.). The elementalcomposition analysis of six discrete spots in the matrix metal indicatedthat copper was present in the matrix metal of the formed metal matrixcomposite body.

EXAMPLE 5

A metal matrix composite body was formed in a manner substantiallyidentical to that described for the metal matrix composite of Example 2.

The formed metal matrix composite body was modified by heating within abed of alumina and boron oxide. Specifically, the formed metal matrixcomposite body was cut with a diamond saw to form a composite sampleweighing about 87 grams, and the composite sample was placed into analumina boat. The composite sample was then surrounded withapproximately 144 grams of a material comprising, by weight, about 50%boron oxide (Fisher Scientific, Pittsburgh, Pa.) and about 50% alumina.A layer of boron oxide, weighing about 96 grams, was then placed on topof the alumina-boron oxide material surrounding the composite body.

The alumina boat and its contents were placed into an air atmosphere boxfurnace at room temperature. The temperature of the furnace wasincreased to about 850° C. at a rate of about 300° C. per hour, held atabout 850° C. for about 150 hours, then decreased to room temperature ata rate of about 300° C. per hour.

The alumina boat and its contents were removed from the furnace, and thecomposite sample was sectioned, mounted, and polished for examinationunder an optical microscope. It was noted that a reaction layer 71 hadformed around the perimeter of the alumina grains 72, as shown in FIG.6.

EXAMPLE 6

This Example demonstrates the spontaneous infiltration of a moltenmatrix metal into a preform containing as an additive a second materialwhich may modify at least one property of a metal matrix composite body.A schematic view of the lay-up employed in carrying out the spontaneousinfiltration process of the present Example is shown in cross-section inFIG. 7.

A series of preforms containing a filler material and variouspercentages of second materials were fabricated as follows. About 5% byweight of Shamrock 642 CERACER™ binder (Shamrock Technologies, Inc.,Newark, N.J.) was added to a permeable mass comprising about 3% byweight AESAR® magnesium particulate (-325 mesh), Aesar Group of JohnsonMatthey Co., Seabrook, N.H.) and the balance 320 grit TETRABOR® boroncarbide particulate (average particle diameter of about 32 microns, ESKEngineered Materials, New Canaan, Conn.) or Grade LC12N silicon nitrideparticulate (Hermann C. Starck, Inc., New York, N.Y.) having an averageparticle size of about 1-2 microns and/or a 70:30 weight ratio of 220and 500 grit 39 CRYSTOLON® silicon carbide particulates (averageparticle diameters of about 66 and 17 microns, respectively, Norton Co.,Worcester, Mass.). The mixture was homogenized by roll mixing in aplastic jar on a mill rack for about 16 hours. About 50% of the weightof the mixture was present as 1/8 inch (3 mm) to 3/16 inch (5 mm)diameter ceramic beads to help disperse the constituents of the mixture.A cylindrical preform measuring about 1 inch (25 mm) in diameter byabout 0.5 inch (13 mm) thick was uniaxially dry pressed under a maximumapplied pressure of about 3.9 Ksi (27 MPa).

The compositional differences among the various preforms was as follows.The Sample F preform contained about 5% by weight of the above-describedboron carbide particulate. The Sample G preform contained about 10% byweight boron carbide particulate. The Sample H preform contained about25% by weight boron carbide particulate. The Sample I preform containedabout 50% by weight boron carbide particulate. The Sample K preformcontained about 5% by weight of the above-described silicon nitrideparticulate. The Sample L preform contained about 10% by weight siliconnitride particulate. The Sample M preform contained about 25% by weightsilicon nitride particulate, and the Sample N preform contained about50% by weight silicon nitride particulate. The Sample E and Sample Jpreforms did not contain any boron carbide or silicon nitrideparticulates.

Lay-ups for each of the Sample E-N preforms were then fabricated asfollows. Referring to FIG. 7, an alumina sagger tray 100 measuring about4 inches (102 mm) long by about 2 inches (51 mm) wide by about 0.5 inch(13 mm) high was lined in its interior with a single sheet of GRAFOIL®graphite foil (Union Carbide Co., Carbon Products Division, Cleveland,Ohio) having a thickness of about 15 mils (0.38 mm) by makingstrategically placed cuts and folds in the graphite foil and cementingthe folds to one another using RIGIDLOCK® graphite cement (PolycarbonCorp., Valencia, Calif.) to form a box. The formed graphite foil box 102had substantially the same length and width dimensions as the interiorof the alumina sagger tray 100, but had a height of about 1.5 inches (37mm). Each preform 104 was placed in the center of the base of eachformed graphite foil box 102. A bedding or barrier material 106comprising by weight about 9% borosilicate glass frit (Grade F-69,Fusion Ceramics, Inc., Carrollton, Ohio) and the balance 90 grit 38ALUNDUM® alumina particulate (Norton Co., Worcester, Mass.) having anaverage particle sizes of about 216 microns was poured into the graphitefoil box 102 around the preform 104 to a level substantially flush withthe top surface of the preform 104. The top of the preform 104 was sweptclean of any of this bedding material admixture 106 and about 0.14 gramsof magnesium particulate 108 (-50 mesh, atomized, Hart Corporation,Tamaqua, Pa.) having substantially all particles smaller than about 300microns in diameter was sprinkled evenly over the top surface of thepreform 104. An ingot of a matrix metal comprising by weight about 15%silicon, about 0.5% magnesium and the balance aluminum and weighingabout 44 grams and measuring about 2 inches (51 mm) long by about 1 inch(25 mm) wide by about 0.5 inch (13 mm) high was placed squarely on topof the layer of magnesium particulate 108. Additional alumina/glass fritparticulate bedding material 106 was the poured into the graphite foilbox 102 around and over the ingot of matrix metal 110 substantiallycompletely burying the preform 104, the magnesium particulate 108 andthe ingot of matrix metal 110, thus completing the lay-up.

Each lay-up comprising the alumina sagger tray 100 and its contents wasthen placed into a controlled atmosphere furnace at about roomtemperature. The furnace chamber was evacuated to a vacuum of about 30inches (762 mm) of mercury and then backfilled with commercially purenitrogen gas to substantially atmospheric pressure. A nitrogen gas flowrate of about 500 standard cubic centimeters per minute (sccm) wasthereafter established and maintained. The furnace temperature was thenincreased from about room temperature to a temperature of about 400° C.at a rate of about 200° C. per hour. After maintaining a temperature ofabout 400° C. for about 2 hours, the furnace temperature was thenfurther increased to a temperature of about 800° C. at a rate of about200° C. per hour. After maintaining a temperature of about 800° C. forabout 20 hours, power to the heating elements of the furnace wasinterrupted and the furnace and its contents were allowed to cool at thefurnace cooling rate back to about room temperature. After cooling tosubstantially room temperature, the lay-up was removed from thecontrolled atmosphere furnace. The alumina/glass frit particulatebedding material was removed with light hammer blows to reveal that thematrix metal had at least partially infiltrated the dry pressed preformto produce a metal matrix composite body.

Each of the formed metal matrix composite bodies was then sectioned andpolished to allow hardness indentation measurements and metallographicexaminations to be made.

FIG. 8 is a graph showing the resulting Rockwell A hardness of each ofthe sectioned and polished composite bodies as a function of the typeand amount of material added to the preform which was spontaneouslyinfiltrated by the molten matrix metal in the metal matrix compositeformation process. The data clearly show a strong increase in hardnessas a function of the concentration of the second material in thepreform, with silicon nitride having a more positive response than boroncarbide. The numerical data from FIG. 8 are shown in Table II below.

                  TABLE II                                                        ______________________________________                                        Sample     Second Material                                                                            Hardness (R.sub.A)                                    ______________________________________                                        0 -                     51.5 ± 1.8                                         F           5 wt % B.sub.4 C                                                                          58.0 ± 3.4                                         G          10 wt % B.sub.4 C                                                                          60.5 ± 3.6                                         H          25 wt % B.sub.4 C                                                                          62.9 ± 3.0                                         I          50 wt % B.sub.4 C                                                                          69.4 ± 2.0                                         J                                                                             0 -        48.8 ± 4.0                                                      K           5 wt % Si.sub.3 N.sub.4                                                                   64.5 ± 3.1                                         L          10 wt % Si.sub.3 N.sub.4                                                                   62.1 ± 6.1                                         M          25 wt % Si.sub.3 N.sub.4                                                                   69.5 ± 3.9                                         N          50 wt % Si.sub.3 N.sub.4                                                                   80.3 ± 2.5                                         ______________________________________                                    

Thus, this Example demonstrates that preforms comprising fillers andsecond materials may be spontaneously infiltrated with molten matrixmetals to form metal matrix composite bodies. Furthermore, this Exampledemonstrates that the addition of such second materials may affect atleast one property of the formed metal matrix composite body.Specifically, this Example demonstrates that the hardness of a metalmatrix composite body may be increased by adding boron carbide orsilicon nitride particulate to a silicon carbide particulate preform.

EXAMPLE 7

This Example demonstrates the effect of infiltration temperature on thehardness of a metal matrix composite body formed by spontaneouslyinfiltrating a molten aluminum alloy matrix metal into a permeable masscomprising silicon nitride. FIG. 9 is a cross-sectional schematic viewof the lay-up employed in fabricating the metal matrix composite bodiesof the present Example.

A particulate mixture for dry pressing was fabricated as follows. About2.3% by weight polyvinyl alcohol, about 1.2% by weight PEG 400polyethylene glycol (Union Carbide Co., Danbury, Conn.), about 7.8% byweight NYACOL® 2040 colloidal silica (Nyacol Products, Inc., anaffiliate of PQ Corporation, Ashland, Mass.) and the balance utilitygrade silicon nitride particulate (-325 mesh, Elkem Materials, Inc.,Pittsburgh, Pa.) having substantially all particles smaller than about45 microns in diameter was blended in a high speed, high intensity mixer(Eirich Machines, Inc., Uniontown, Pa.) until the mixture wassubstantially completely homogeneously mixed. The total moisture contentof the mix was about 12.9% by weight. The mixed powder was then allowedto dry in air at about room temperature for about 16 hours. The driedpowder was then passed through a 25 mesh screen (screen opening size ofabout 710 microns) and uniaxially dry pressed at a maximum appliedpressure of about 20 ksi (140 MPa). The dry pressed preform measuredabout 2 inches (51 mm) square by about 0.5 inch (13 mm) thick. The drypressed preform was then fired in a nitrogen atmosphere to a temperatureof about 850° C., which temperature was maintained for about 4 hours.The furnace and the fired preform contained therein were allowed to coolback to about room temperature at the natural furnace cooling rate.

Referring to FIG. 9, a lay-up was prepared as follows. A sheet ofGRAFOIL® graphite foil 120 (Union Carbide Co., Carbon Products Division,Cleveland, Ohio) having substantially the same length and widthdimensions as those of the interior of a Grade ATJ graphite boat 122(Union Carbide Co.) having interior dimensions of about 9 inches (229mm) in length by about 4 inches (102 mm) in width and having a height ofabout 2 inches (51 mm). The pressed and fired preform 124 was thenplaced on top of the sheet of graphite foil 120. A bedding or barriermaterial 126 comprising a particulate mixture comprising about 5% byweight Grade F-69 glass frit (Fusion Ceramics, Inc., Carrollton, Ohio)and the balance 90 grit 38 ALUNDUM® alumina particulate (Norton Co.,Worcester, Mass.) having an average particle size of about 216 micronswas poured into the graphite boat 122 on top of the graphite foil sheet120 around the pressed and fired preform 124 to a level substantiallyflush the top surface of the preform 124. A slurry comprisingapproximately equal weight measures of nickel particulate (-325 mesh,substantially all particle diameters smaller than about 45 microns) andethyl alcohol was painted over the exposed top surface of the preform.The ethyl alcohol was allowed to evaporate leaving a thin coating ofnickel particulate 127 on the top surface of the preform 124. An ingotof a matrix metal 130 comprising by weight about 5% magnesium and thebalance aluminum, weighing about 113 grams and measuring about 2 inches(51 mm) square by about 0.5 inch (13 mm) thick was placed squarely ontop of the nickel coated preform 124. Additional alumina/glass fritparticulate bedding material 126 was then poured into the graphite boat122 around the nickel coated preform 124 and the ingot of matrix metal130, substantially completely burying both to complete the lay-up.

Sample O

The Sample O lay-up comprising the graphite boat 122 and its contentswere then placed into a resistance heated controlled atmosphere furnace.The furnace chamber was evacuated to a vacuum of about 30 inches (762mm) of mercury and then backfilled with commercially pure nitrogen gasto substantially atmospheric pressure. A nitrogen gas flow rate throughthe furnace chamber of about 5 standard liters per minute (slm) wasthereafter established and maintained. The furnace temperature was thenincreased from about room temperature to a temperature of about 800° C.at a rate of about 200° C. per hour. After maintaining a temperature ofabout 800° C. for about 20 hours, the furnace temperature was thendecreased to a temperature of about 700° C. at a rate of about 200° C.per hour. At a temperature of about 700° C., the graphite boat and itscontents were removed from the furnace and placed onto a roomtemperature graphite chill plate to directionally solidify the moltenmatrix metal. After the lay-up had cooled to substantially roomtemperature, the lay-up was disassembled to reveal that the matrix metalhad infiltrated the preform comprising the silicon nitride particulateto form a metal matrix composite body.

Samples P and Q

Metal matrix composite Samples P and Q were formed in substantially thesame manner as was metal matrix composite Sample O, except that thespontaneous infiltration of molten matrix metal into the preformcomprising the silicon nitride particulate was carried at temperaturesof about 900° C. and about 1000° C., respectively.

The formed metal matrix composite bodies were sectioned using a diamondsaw and hardness tested. Table III shows the resulting Rockwell Ahardness for each of Samples O, P and Q as a function of the temperatureat which spontaneous infiltration was conducted. The data clearly showthat increases in the temperature at which infiltration of the moltenaluminum alloy matrix metal is conducted results in increases inhardness of the formed metal matrix composite bodies.

                  TABLE III                                                       ______________________________________                                        Sample   Infiltration Temp. (C.°)                                                               Hardness (R.sub.A)                                   ______________________________________                                        O        800             73.0 ± 1.2                                        P        900             79.7 ± 1.1                                        Q        1000            84.5 ± 1.2                                        ______________________________________                                    

Thus, this Example demonstrates that preforms comprising silicon nitrideparticulate may be spontaneously infiltrated by a molten matrix metal toform a metal matrix composite body. Furthermore, this Exampledemonstrates that the hardness of the resulting metal matrix compositebody is a function of the spontaneous infiltration temperature.Specifically, for the above-described system, higher infiltrationtemperatures produce harder metal matrix composite bodies.

EXAMPLE 8

This Example demonstrates that the properties and the composition of ametal matrix composite body may be modified by a heat treatment which issubsequent to the spontaneous infiltration process employed infabricating the metal matrix composite body. FIG. 10 is across-sectional schematic view of the lay-up employed in fabricating themetal matrix composite body of the present Example.

A preform was fabricated in substantially the same manner as wasdescribed in Example 7.

The preform was then coated with a barrier material comprising colloidalgraphite as follows. A mixture comprising by volume about 50% DAG® 154colloidal graphite (Acheson Colloids, Port Huron, Mich.) and about 50%denatured ethyl alcohol was prepared. A thin layer of this mixture wasthen applied by means of an airbrush on five sides of the preform,leaving one of the 4 inch (102 mm) square faces uncoated. After a shortperiod of time during which substantially all of the ethyl alcoholevaporated, a second such layer of colloidal graphite was applied. Thecoating procedure was performed a third time. The preform coated withthis colloidal graphite mixture was then allowed to dry.

A lay-up was then fabricated as follows. A GRAFOIL® graphite foil sheet120 (Union Carbide Co., Carbon Products Division, Cleveland, Ohio)having substantially the same length and width dimensions as theinterior of a Grade ATJ graphite boat 122 (Union Carbide Co.) measuringin its interior about 0.12 inches (305 mm) long by about 9 inches (229mm) wide by about 4 inches (102 mm) in height. A bedding or barriermaterial comprising by weight about 10% Grade F-69 glass frit (FusionCeramics, Inc., Carrollton, Ohio) and the balance 90 grit 38 ALUNDUM®alumina particulate (average particle size of about 216 microns, NortonCo., Worcester, Mass.) was poured into the graphite boat 122 to a depthof about 1 inch (25 mm) and leveled. Two ingots of a matrix metal 132comprising by weight about 15% silicon, 5% magnesium, 10% copper and thebalance aluminum were placed adjacent to one another and on top of theleveled alumina/glass fit particulate bedding admixture 126 to form,during subsequent processing at elevated temperature, one large ingothaving dimensions of about 10 inches (254 mm) in length by about 7inches (178 mm) in width by about 0.5 inch (13 mm) thick. A GRAFOIL®graphite foil sheet 134 (Union Carbide Co.) having a thickness of about15 mils (0.38 mm), length and width dimensions substantially matchingthose of the assembled ingots of matrix metal 132 and a square openinghaving length and width dimensions approximately 0.375 inch (10 mm)shorter than each of the length and width dimensions of the preform 124was placed squarely on top of the assembled ingots of matrix metal 132.About 2.25 grams of magnesium particulate 128 (-50 mesh, atomized, HartCorp., Tamaqua, Pa.) having substantially all particles smaller thanabout 300 microns in diameter was sprinkled evenly over the exposedsurface of the assembled matrix metal ingots 132 defined by the openingin the graphite foil window 134. The preform 124 was then placedsquarely over the magnesium particulate layer 128 and aligned with thesquare opening in the graphite foil window 134. Additional alumina/glassfrit particulate bedding material 126 was then poured in the graphiteboat 122 around the assembled ingots of matrix metal 132 to a level justabove the bottom surface of the preform 124 to complete the lay-up.

The lay-up comprising the graphite boat 122 and its contents was thenplaced into a controlled atmosphere furnace at about room temperature.The furnace chamber was evacuated to a vacuum of about 30 inches (762mm) of mercury and then backfilled with commercially pure nitrogen gasto about atmospheric pressure. A nitrogen gas flow rate of about 5 slmthrough the furnace chamber was thereafter established and maintained.The furnace temperature was then increased from about room temperatureto a temperature of about 250° C. at a rate of about 200° C. per hour.After maintaining a temperature of about 250° C. for about 26.5 hours,the temperature was then increased to a temperature of about 950° C. ata rate of about 200° C. per hour. After maintaining a temperature ofabout 950° C. for about 30 hours, the temperature was then decreased toa temperature of about 700° C. at a rate of about 200° C. per hour. At atemperature of about 700° C., the furnace chamber was opened and thelay-up was removed. The graphite-coated preform was removed from thepool of molten matrix metal and residual adhered metal matrix wasscraped off of the bottom surface of the preform. The preform was thenallowed to cool in air at ambient temperature to substantially roomtemperature.

After the preform had cooled to substantially room temperature, thecolloidal graphite coating was removed by grit blasting to reveal thatthe matrix metal had substantially completely infiltrated the siliconnitride preform to form a metal matrix composite body. A portion of theformed metal matrix composite body was removed by means of a diamond sawand characterized in terms of its hardness and chemical composition.Specifically, the sectioned sample was hardness tested on the Rockwell Ascale and chemically analyzed using qualitative x-ray diffractionanalysis.

For the qualitative x-ray diffraction analysis, a portion of thesectioned sample was ground to a fine powder in a mortar and pestle andplaced into the sample chamber of a Model D500 x-ray diffractometer(Siemens AG, Munich, Germany). The sample was then scanned withunfiltered with Cu_(K) alpha x-radiation at an energy of about 40 KeV.The counting time was about 2 seconds at each 0.03° interval of 2-theta.

The remainder of the formed metal matrix composite was then given apost-formation process treatment, specifically comprising an elevatedtemperature heat treatment. Reference is made to FIG. 13, which shows asimilar lay-up to the lay-up described below for conducting thispost-formation process treatment.

A boat 148 made out of a castable refractory was filled to a depth ofabout 1 inch (25 mm) with wollastonite powder 150 (-325 mesh, NycoCorp., Willsboro, N.Y.) having substantially all particle diameterssmaller than about 45 microns. The formed metal matrix composite body152 was then placed into the refractory boat 148 on top of the layer ofwollastonite powder 150 such that the metal matrix composite body 152contacted the wollastonite powder 150. Additional wollastonite powder150 (-325 mesh) was then poured into the refractory boat 148substantially completely burying the metal matrix composite body 152 toa depth of about 1 inch (25 mm). The refractory boat and its contentswas then placed into an air atmosphere furnace at about 20° C. Thefurnace and its contents were then heated from about 20° C. to atemperature of about 1200° C. at a rate of about 50° C. per hour. Aftermaintaining a temperature of about 1200° C. for about 24 hours, thefurnace temperature was then decreased to about 20° C. at a rate ofabout 100° C. per hour. After the furnace had cooled substantially toabout 20° C., the refractory boat 148 and its contents was removed fromthe furnace and the metal matrix composite body 152 was recovered fromthe refractory boat 148. This heat treated metal matrix composite bodywas then characterized in terms of hardness and chemical composition insubstantially the same manner as for the non-heat treated portion of themetal matrix composite body. The results are shown in Table IV. The dataclearly show that the heat treated metal matrix composite body is harderthan the same body before the heat treatment. Moreover, the relativex-ray intensities reported in the Table show that the heat treatmentprocess results in a decrease in the amount of silicon nitride andaluminum and an increase in the amount of aluminum nitride present inthe body. These data suggest that the heat treatment process may permitthe aluminum matrix metal and the silicon nitride reinforcement to reactto form aluminum nitride above and beyond the amount of aluminum nitridewhich formed as a result of the initial infiltration of molten aluminummatrix metal into the silicon nitride preform. FIGS. 11a and 11b areoptical photomicrographs taken at about 400× magnification of thepolished cross-sections of the as-formed and heat-treated metal matrixcomposite bodies of the present Example.

                  TABLE IV                                                        ______________________________________                                                   As Infiltrated                                                                         After Heat Treatment                                      ______________________________________                                        Hardness (R.sub.A)                                                                         84         87                                                    Composition                                                                   (as suggested by                                                              relative x-ray                                                                intensities):                                                                 α-Si.sub.3 N.sub.4                                                                   30         10                                                    β-Si.sub.3 N.sub.4                                                                    25         10                                                    AlN          35         70                                                    Al           20          0                                                    Si           100        100                                                   ______________________________________                                    

This Example therefore demonstrates that a preform comprising siliconnitride particulate may be spontaneously infiltrated by a moltenaluminum matrix metal and thereafter heat treated to modify thecomposition and at least one property of the metal matrix compositebody.

EXAMPLE 9

This Example demonstrates the fabrication of a number of metal matrixcomposite bodies by the spontaneous infiltration of a variety ofaluminum matrix metal alloys into permeable masses comprising severalcombinations of silicon carbide and/or silicon nitride. This Examplefurthermore demonstrates a post-formation process treatment (e.g., aheat treatment) given to the formed metal matrix composite bodies. Thelay-up employed in fabricating the metal matrix composite bodies isillustrated in a cross-sectional schematic view in FIG. 12.

Permeable preforms representing Samples R-Y were fabricated by uniaxialdry pressing. A dry pressable powder blend was fabricated as follows.About 3000 grams of the silicon carbide and/or silicon nitrideparticulate was placed into the mixing chamber of a high speed, highintensity, high shear mixer (Eirich Machines, Inc., Uniontown, Pa.).About 585 grams of a binder solution was divided into threeapproximately equal quantities. The first fraction was added to themixing chamber and mixing was commenced. After mixing for about 1minute, the mixer was turned off, the chamber was opened and thatmixture which was adhered to the mixing chamber walls and the blades wasscraped off with a spatula and deposited back into the mixing chamber.This procedure was repeated for the second and final remaining binderfractions. Mixing was then continued until the binder appeared to beuniformly dispersed into the powder and the mixture was appropriatelygranulated for subsequent dry pressing. The granulated powder was thenpoured out onto a sheet of paper and allowed to dry in air at about roomtemperature for about 16 hours. The dried powder was then screenedthrough an approximately 25 mesh screen (screen openings of about 710microns). The dried and screened powder was then loaded into a pressingdie to a particular volume and, after a de-airing procedure consistingessentially of twice gently loading and unloading the powder, the powderwas uniaxially pressed in a floating die arrangement under an appliedpressure of about 20 ksi (140 MPa). The resulting preforms which wererecovered from the die had dimensions of about 4 inches (102 mm) squareby about 1.20 inches (30.5 mm) to about 1.24 inches (31.5 mm) thick.

Samples R and S

The particulate material making up the Sample R and Sample S permeablemasses comprised utility grade silicon nitride particulate (ElkemMaterials, Inc., Pittsburgh, Pa.) having substantially all particlessmaller than about 45 microns in diameter. Moreover, the binder solutionemployed in preparing a dry pressable mixture comprised by weight about40% of a polyvinyl alcohol stock solution, about 6% polyethylene glycol(400 molecular weight), about 40% NYACOL® 2040 colloidal silicasuspension (Nyacol Products, Inc., an affiliate of PQ Corp., Ashland,Mass.) and about 14% water. The polyvinyl alcohol stock solutioncomprised by weight about 30% polyvinyl alcohol dissolved in water. Atotal of about 585 grams of the above-described binder components weremixed in a plastic jug having a volume of about 1 gallon (3.8 liters)using a low speed electric paddle mixer. Mixing ceased when the solutionbegan to foam slightly.

Samples T and U

The permeable masses of Samples T and U comprised utility grade siliconnitride particulate (Elkem Materials, Inc.) having substantially allparticle diameters smaller than about 75 microns. Furthermore, thebinder composition was substantially the same as that employed in makingthe press mixture for Samples R and S above.

Samples V and W

The permeable masses for Samples V and W comprised a blend of siliconnitride and silicon carbide particulates. Specifically, the permeablemasses comprised by weight about 50% utility grade silicon nitrideparticulate (Elkem Materials, Inc.) having substantially all particlediameters between about 23 microns and about 45 microns and the balance500 grit 39 CRYSTOLON® SUPER STRONG silicon carbide particulate (averageparticle diameter of about 17 microns, Norton Co., Worcester, Mass.).The above-described particle size distribution for the silicon nitrideparticulate was created by preparing an aqueous slurry of as-receivedsilicon nitride particulate (-325 mesh, Elkem Materials, Inc.) havingsubstantially all particles smaller than about 45 microns in diameter,allowing the particulates comprising the slurry to settle in accordancewith Stokes' Law and decanting off the remaining suspension of water andfine particulate silicon nitride. Subsequent analysis of the particlesize distribution of the settled particulate material using aMICROMERITICS® SEDIGRAPH 5100 particle size analyzer (MicromeriticsInstrument Corp., Norcross, Ga.) revealed that a settling time of about0.5 hour was sufficient to substantially completely eliminate theparticles smaller than about 23 microns in diameter from the as-receivedsilicon nitride powder.

Unlike Samples R-U, however, the binder system employed for fabricatingthe Sample V and W preforms comprised by weight about 51.6% of thepreviously described polyvinyl alcohol stock solution, about 9.7% of the400 molecular weight polyethylene glycol, and the balance NYACOL® 2040colloidal silica suspension (Nyacol Products, Inc.)

Samples X and Y

The Sample X and Y preforms featured a particulate blend comprising byweight about 50% silicon nitride particulate (Elkem Materials, Inc.)having substantially all particles smaller than about 45 microns indiameter and about 50% 500 grit 39 CRYSTOLON® SUPER STRONG siliconcarbide particulate (average particle diameter of about 17 microns,Norton Co.). The binder system employed in fabricating the press mix forthe Sample X and Y preforms was substantially the same as that employedfor fabricating the Sample R-U preforms.

The dry pressed preforms were then debindered. Specifically, eachpreform was placed on an about 1/8 inch (3 mm) thick sheet ofCARBORUNDUM® FIBERFRAX® ceramic paper which in turn was supported by asilicon carbide plate. The silicon carbide plate and its contents werethen placed into an air atmosphere furnace and subjected to thefollowing heating schedule. The preforms were heated in air from about20° C. to a temperature of about 500° C. at a rate of about 50° C. perhour. After maintaining a temperature of about 500° C. for about 6 hoursin air, the temperature was then increased to a temperature of about1000° C. at a rate of about 100° C. per hour. After maintaining atemperature of about 1000° C. for about 4 hours in air, the temperaturewas then decreased to about 20° C. at a rate of about 100° C. per hour.

Three coats of DAG® 154 colloidal graphite slurry (Acheson Colloids Co.,Port Huron, Mich.) was coated by means of a foam paintbrush onto fivesides of each preform in substantially the same manner as was describedin Example 8, leaving one of the 4 inch (102 mm) square faces uncoated.A lay-up was then fabricated as follows. Referring to FIG. 12, anapproximately 16 inch (406 mm) square graphite box 140 approximately 10inches (254 mm) high and open on both ends was placed onto a graphiteplate 142. The five enclosed sides of the box were then lined with asingle sheet of GRAFOIL® graphite foil 120 (Union Carbide Co., CarbonProducts Div., Cleveland, Ohio) having a thickness of about 15 mils(0.38 mm) by making strategically placed cuts and folds in the graphitefoil sheet 120 to make a box. A particulate bedding admixture 126comprising by weight about 10% F-69 glass frit (Fusion Ceramics, Inc.,Carrollton, Ohio) and the balance 90 grit 38 ALUNDUM® aluminaparticulate (average particle size of about 216 microns, Norton Co.,Worcester, Mass.) was poured into the graphite foil box 120 to a depthof about 1 inch (25 mm). The colloidal graphite 136 coated preforms 124were then placed into the graphite foil box 120 on top of theparticulate bedding admixture 126 with the uncoated side facing up.Samples R-Y comprised 8 of the 9 preform tiles so placed in graphitefoil box 120. Magnesium particulate 128 (-50 mesh, atomized, Hart Corp.,Tamaqua, Pa.) having substantially all particles smaller than about 300microns in diameter was sprinkled evenly over the exposed surface ofeach preform 124 at a concentration of about 23 mg per squarecentimeter. A gating means comprising a sheet 134 of GRAFOIL® graphitefoil having a thickness of about 15 mils (0.38 mm) and length and widthdimensions substantially matching those of the preform and having a holein its center of about 3/16 inch (5 mm) in diameter was then placed ontoa sheet of metal 146 comprising by weight about 0.4-0.8% Si, <0.7% Fe,0.15-0.40% Cu, <0.15% Mn, 0.8-1.2% Mg, 0.04-0.35% Cr, <0.25% Zn and thebalance Al (Aluminum Association Alloy No. 6061) having a thickness ofabout 23 mils (0.6 mm) and overall dimensions substantially matchingthose of the preform 124. This assembly was then placed on top of thelayer of magnesium particulate 128 on top of the preform 124 with themetal sheet 146 contacting the magnesium particulate 128. The hole inthe graphite foil sheet 134 was then filled with additional magnesiumparticulate 128 (-50 mesh, Hart Corp.). An ingot of matrix metal 144measuring about 4 inches (102 mm) square and weighing about 730 gramsand having a composition as described in Table V was placed over each ofthe graphite foil sheet gating means 134. Additional particulate beddingmaterial 126 was then poured into the graphite foil box 120substantially completely burying each matrix metal ingot 144 to a depthof about 1 inch (25 mm) to complete the lay-up.

                  TABLE V                                                         ______________________________________                                        Sample   Powder Blend in Preform                                                                         Matrix Metal                                       ______________________________________                                        R        -45 μm Si.sub.3 N.sub.4                                                                      Al-5 Mg                                            S        -45 μm Si.sub.3 N.sub.4                                                                      Al-10 Si-5 Mg                                      T        -200 mesh Si.sub.3 N.sub.4                                                                      Al-5 Mg                                            U        -200 mesh Si.sub.3 N.sub.4                                                                      Al-10 Si-5 Mg                                      V        50 wt % +23 μm Si.sub.3 N.sub.4                                                              Al-10 Si-5 Mg                                               50 wt % 500 grit SiC                                                 W        50 wt % +23 μm Si.sub.3 N.sub.4                                                              Al-12.5 Si-5 Mg                                             50 wt % 500 grit SiC                                                 X        50 wt % -45 μm Si.sub.3 N.sub.4                                                              Al-10 Si-5 Mg                                               50 wt % 500 grit SiC                                                 Y        50 wt % -45 μm Si.sub.3 N.sub.4                                                              Al-12.5 Si-5 Mg                                             50 wt % 500 grit SiC                                                 ______________________________________                                    

The lay-up was then placed into a controlled atmosphere furnace at about20° C. The furnace door was closed and the furnace chamber atmospherewas evacuated to about 30 inches (762 mm) of mercury vacuum and thenbackfilled with commercially pure nitrogen gas to about atmosphericpressure. A nitrogen gas flow rate of about 250 cubic feet per hour (118liters per minute) through the furnace chamber was thereafterestablished and maintained. The furnace temperature was then increasedfrom about 20° C. to a temperature of about 200° C. at a rate of about100° C. per hour. After maintaining a temperature of 200° C. for about 2hours, the furnace temperature was then increased to a temperature ofabout 800° C. at a rate of about 100° C. per hour. After maintaining atemperature of about 800° C. for about 45 hours, the temperature wasthen decreased to about 20° C. at a rate of about 100° C. per hour.After the furnace chamber had cooled to about 20° C., the furnacechamber was opened, the lay-up was retrieved and disassembled.Specifically, the particulate bedding material was removed using lighthammer blows to reveal that the matrix metal ingots had at leastpartially spontaneously infiltrated the preforms to form metal matrixcomposite bodies having some residual uninfiltrated matrix metalremaining attached to one side of the formed metal matrix compositebody.

Referring to FIG. 13, the formed metal matrix composite bodies were thenheat treated in substantially the same manner as the metal matrixcomposite body of Example 8 with the exception that the residual matrixmetal 144 was left attached to the metal matrix composite bodies duringthe heat treatment to provide a reservoir of metal to counter thepropensity for shrinkage porosity to otherwise form during thispost-processing.

Knoop microhardness measurements were taken on heat treated Samples V, Wand X. Conversion of the Knoop microhardness values to the Rockwell Ahardness scale was done using the hardness conversion chart found onpage 122 of the ASM Metals Reference Book, Second Edition. Both Knoopand Rockwell A hardness values are reported in Table VI.

This Example therefore demonstrates the fabrication of a variety ofmetal matrix composite bodies comprising various permeable masses,spontaneously infiltrated by various matrix metals to form metal matrixcomposite bodies which may thereafter be heat treated at an elevatedtemperature. The Example furthermore demonstrates the concept ofsupplying additional matrix metal to the formed metal matrix compositebody during the post-formation treatment process.

                  TABLE VI                                                        ______________________________________                                        Sample   Knoop Microhardness                                                                          Rockwell A Hardness                                   ______________________________________                                        V        775 ± 53    81.9                                                  W        753 ± 55    81.8                                                  X         865 ± 113  84.4                                                  ______________________________________                                    

EXAMPLE 10

This Example demonstrates the fabrication of a heat treatable metalmatrix composite body. In particular, this Example demonstrates thefabrication of a composite body by means of spontaneously infiltrating apermeable mass comprising a plurality of second materials with a moltenmatrix metal and subsequently heating the infiltrated mass to atemperature above the infiltration temperature so as to react the secondmaterials to form a variety of reaction products. The permeable mass ofthe present example does not initially contain any substantiallynon-reactive filler material; however, the present Example alsodemonstrates the formation of such a reinforcement filler wherein thissubstantially non-reactive reinforcement filler forms in situ as aresult of the heat treatment process.

Most of the geometry of the lay-up employed in carrying out thespontaneous infiltration process of the present Example is shownschematically in cross-section in FIG. 12.

A series of preforms containing various ratios of second materials werefabricated as follows.

Sample AA

First a dry pressable powder mixture was prepared. Specifically, into anapproximately 1 quart (0.95 liter) plastic jar was placed a mixturecomprising by weight about 3 percent ground magnesium particulate (-325mesh, Hart Corp., Tamaqua, Pa.) having substantially all particlessmaller than about 45 microns in diameter and the balance comprising asecond material which consisted essentially of utility grade siliconnitride particulate (-325 mesh, Elkem Materials, Inc., Pittsburgh, Pa.)having substantially all particles smaller than about 45 microns indiameter. About 5 percent of the weight of this mixture was added asCERACER® 642 free-flowing PT wax binder (Shamrock Corp., Newark, N.J.)was added to the mixture as a binder to yield between about 50 and 75grams of raw material for dry pressing a preform. About half of theweight of this raw material, was then added to the contents of theplastic jar in the form of BURUNDUM® cylindrical ceramic grinding media(U.S. Stoneware, Mahwah, N.J.) each cylinder measuring about 1/2 inch(13 mm) in height by about 1/2 inch (13 mm) in diameter. The plastic jarwas then sealed with its lid and the jar and its contents were thenplaced onto a rotating mill rack for about 16 hours to roll mix thecontents of the jar. The roll-mixed powder was then passed through a 25mesh screen (opening size of about 710 microns) separate the ceramicbeads from the powder. The roll mixed powder was then placed into asteel pressing die and a preform tile measuring about 2 inches (51 mm)square by about 0.5 inch (13 mm) thick was uniaxially dry pressed undera maximum applied pressure of about 10 ksi (69 MPa).

Sample BB

Sample BB was prepared in substantially the same manner as was Sample AAwith the exception that the second material itself comprised a mixture.Specifically, the second material for Sample BB comprised by weightabout 25.8 percent 320 grit TETRABOR® boron carbide particulate (ESKEngineered Materials, Inc., New Canaan, Conn.) having an averageparticle diameter of about 32 microns and the balance the utility gradesilicon nitride particulate (-325 mesh, Elkem Materials, Inc.).

Sample CC

Sample CC was prepared in substantially the same manner as was Sample BBwith the exception that the second material comprised by weight about51.5 percent of the 320 grit TETRABOR® boron carbide particulate (ESKEngineered Materials, Inc.) and the balance the utility grade siliconnitride particulate (-325 mesh, Elkem Materials, Inc.).

Sample DD

Sample DD was prepared in substantially the same manner as was Sample BBwith the exception that the second material comprised by weight about77.3 percent of the 320 grit TETRABOR® boron carbide particulate (ESKEngineered Materials, Inc.) and the balance the utility grade siliconnitride particulate (-325 mesh, Elkem Materials, Inc.).

Sample EE

The Sample EE preform was fabricated in substantially the same manner aswas the Sample AA preform with the exception that the second materialcomprised 320 grit TETRABOR® boron carbide particulate (average particlediameter of about 32 microns, ESK Engineered Materials, Inc.) instead ofthe utility grade silicon nitride material.

The preforms were then coated with a colloidal graphite barrier materialin substantially the same manner as was described in Example 8.

A lay-up was then fabricated as follows. The five interior sides of agraphite boat measuring about 9 inches (229 mm) square by about 4 inches(102 mm) high was lined with a single sheet of GRAFOIL® graphite foilmaterial (Union Carbide Co., Carbon Products Division, Cleveland, Ohio)having a thickness of about 15 mils (0.38 mm) by making strategicallyplaced cuts and folds in the graphite foil material. The colloidalgraphite 136 coated preforms 124 were then placed onto the floor of thegraphite foil lined boat with the uncoated side facing up (e.g., towardsthe opening of the boat). Referring to FIG. 12, which adquatelyillustrates the remaining components of the lay-up, a particulateadmixture 126 comprising by weight about 2.5 percent F-69 glass frit(Fusion Ceramics, Inc., Carrollton, Ohio) and the balance 90 grit 38ALUNDUM® aluminum oxide particulate (Norton Co., Worcester, Mass.)having an average particle size of about 216 microns, was poured intothe graphite foil 120 lined boat. Magnesium particulate 128 (-50+100mesh, ground, Hart Corp.) having substantially all particles betweenabout 150 and about 300 microns in diameter was sprinkled evenly overthe exposed surface of each preform 124 until a total of about 0.6 gramsof magnesium particulate had been supplied to the exposed surface ofeach preform 124. A gating means comprising a sheet 134 of GRAFOIL®graphite foil material (Union Carbide Co.) having a thickness of about15 mils (0.38 mm) and measuring about 2.25 inches (57 mm) square andhaving a hole in its center of about 3/16 inch (5 mm) in diameter wasthen placed in rotational alignment flat onto a plate of metal 146comprising by weight about 0.4-0.8% Si, <0.7% iron, 0.15-0.40% Cu,<0.15% Mn, 0.8-1.2% Mg, 0.04-0.35% Cr, <0.25% Zn and the balance Al(Aluminum Association Alloy No. 6061) having a thickness of about 32mils (0.8 mm) and measuring about 2 inches (51 mm) square. This assemblycomprising this sheet 134 of graphite foil contacting this aluminumalloy plate 146 was then placed on top of the layer of magnesiumparticulate 128 on top of the preform 124 with the metal plate 146contacting the magnesium particulate 128. The hole in the graphite foilmaterial 134 was then filled with additional ground magnesiumparticulate 128 (-50+100 mesh, Hart Corp.). An ingot of matrix metal 144measuring about 2 inches (51 mm) square by about 1 inch (25 mm) thickand weighing about 181 grams and comprising by weight about 10% siliconand the balance aluminum was placed over each of the graphite foilgating means 134. a particulate admixture 126 comprising by weight about2.5 percent F-69 glass frit (Fusion Ceramics, Inc., Carrollton, Ohio)and the balance 90 grit 38 ALUNDUM® aluminum oxide particulate (NortonCo., Worcester, Mass.) having an average particle size of about 216microns, was poured into the graphite foil 120 lined boat substantiallycompletely burying each matrix metal ingot 144 to a depth of about 0.5inch (13 mm) to complete the lay-up.

The lay-up was then placed into a controlled atmosphere furnace at atemperature of about 25° C. The furnace chamber was closed and thefurnace chamber atmosphere was evacuated to a final pressure of about 28mm of mercury and then backfilled with commercially pure nitrogen gas toabout atmospheric pressure. A nitrogen gas flow rate of about 5 standardliters per minute (SLPM) was thereafter established and maintainedflowing through the furnace chamber. The furnace temperature was thenincreased from about 25° C. to a temperature of about 400° C. at a rateof about 50° C. per hour. After maintaining a temperature of about 400°C. for about 10 hours, the furnace temperature was then increased to atemperature of about 800° C. at a rate of about 100° C. per hour. Aftermaintaining a temperature of about 800° C. for about 15 hours, thefurnace temperature was decreased to a temperature of about 700° C. at arate of about 200° C. per hour. At a temperature of about 700° C., thefurnace chamber was then opened and the graphite boat and its contentswere removed from the furnace chamber and placed onto a water cooledcopper quench plate to directionally solidify the formed metal matrixcomposite bodies. After the graphite boat and its contents had cooled tosubstantially room temperature (e.g., about 25° C.), the preforms wererecovered from the graphite boat. Inspection of each preform revealedthat each had been at least partially infiltrated with matrix metal toform a metal matrix composite material. Each of the formed metal matrixcomposite bodies had some residual uninfiltrated matrix metal remainingattached to one side of the formed body.

Each of the formed metal matrix composite bodies were then sectioned inhalf using a diamond saw and one of each pair of sectioned bodies wereheat treated in substantially the same manner as was described inExample 9 (i.e., heat treated with the residual body of uninfiltratedmatrix metal remaining attached to the metal matrix composite material)and as illustrated in FIG. 13.

The heat treated and non-heat treated halves of the Sample CC compositematerial were then characterized in terms of chemical composition usingqualitative x-ray diffraction analysis substantially as was described inExample 8. Table VII shows the relative change in concentration of thevarious chemical constituents of the formed metal matrix composite bodyas a result of the heat treatment step. Specifically, Table VII reportsthe relative x-ray intensities for each of the detected substances witheach x-ray analysis being normalized to an intensity of 100 for thelargest x-ray peak. Examination of Table VII suggests a complexinteraction of many of the metal matrix composite constituents as aresult of the elevated temperature heat treatment. Specifically, thedata show an increase of the silicon concentration which may beattributable to reaction of aluminum with silicon nitride; consumptionof silicon nitride to below the x-ray diffraction detection limit; asignificant increase in the concentration of aluminum nitride; and theappearances of silicon carbide and aluminum boride (specifically, AlB₁₀)phases upon heat treatment.

This Example thus further demonstrates the fabrication of a heattreatable metal matrix composite body. In particular, this Exampledemonstrates the fabrication of a composite body by means ofspontaneously infiltrating a permeable mass with a molten matrix metal.The permeable mass

                  TABLE VII                                                       ______________________________________                                        Constituent                                                                   of Composite                                                                              As-Infiltrated                                                                           After Heat-Treatment                                   ______________________________________                                        Al          100        77                                                     Si          19         100                                                    Si.sub.3 N.sub.4                                                                          5                                                                 β-Si.sub.3 N.sub.4                                                                   2                                                                 B.sub.4 C   6          4                                                      AlN         2          53                                                     SiC                    20                                                     AlB.sub.10             11                                                     ______________________________________                                    

consists essentially of materials which are reactive at elevatedtemperatures such that subsequently heating of the infiltrated mass to atemperature above the infiltration temperature causes the materialsmaking up the permeable mass to react to form a variety of reactionproducts. Moreover, this Example demonstrates that one or more suchreaction products may become reactants for an additional, secondaryreaction in what may be a complex sequence of chemical reactions.Finally, the present Example demonstrates that certain substantiallynon-reactive reinforcement filler materials such as silicon carbide maybe absent initially from a metal matrix composite body but subsequentlyformed in situ as a result of an elevated temperature heat treatmentprocess.

While the preceding embodiments have been described with particularityand demonstrate the utility of the invention, various modificationsshould be considered to be within the scope of the claims appendedhereto.

What is claimed is:
 1. A method of forming a metal matrix compositebody, comprising:forming a permeable mass comprising at least one secondmaterial consisting essentially of at least one precursor to a metal;heating a body of molten matrix metal to an infiltrating temperature;infiltrating said molten matrix metal at least partially into saidpermeable mass at said infiltrating temperature to form an at leastpartially infiltrated mass; thereafter heating said at least partiallyinfiltrated mass to a temperature higher than said infiltratingtemperature; forming at least one reaction product in at least a portionof said at least partially infiltrated mass; and cooling said at leastpartially infiltrated mass, at least a portion of which has beenmodified by said reaction product.
 2. The method of claim 1, whereinsaid matrix metal comprises aluminum.
 3. The method of claim 1, furthercomprising at least two second materials, wherein said at least onereaction product comprises a reaction product of at least two of said atleast two second materials.
 4. The method of claim 1, wherein said atleast one reaction product comprises at least one member selected fromthe group consisting of a metallic material and a ceramic material. 5.The method of claim 1, wherein said at least one second materialcomprises at least one member selected from the group consisting ofboron carbide and silicon nitride.
 6. The method of claim 1, whereinsaid infiltrating comprises at least one technique selected from thegroup consisting of spontaneous infiltration, pressure infiltration andvacuum infiltration.
 7. The method of claim 1, further comprisingcontacting said at least partially infiltrated mass to at least onesource of additional matrix metal during said heating of said at leastpartially infiltrated mass.
 8. The method of claim 1, wherein at least aportion of said reaction product forms during said infiltrating step. 9.The method of claim 1, wherein at least a majority of said reactionproduct forms upon heating said at least partially infiltrated mass tosaid higher temperature.
 10. The method of claim 1, wherein said atleast one reaction product comprises a reaction product of said matrixmetal and said at least one second material.
 11. The method of claim 3,wherein a volume of said at least one reaction product is less than thesum of the volumes of said second material and said matrix metalreacting to form said at least one reaction product.
 12. The method ofclaim 1, wherein said infiltrating comprises spontaneous infiltration.13. The method of claim 12, wherein an infiltrating atmospherecommunicates with at least one of said permeable mass and said matrixmetal at least at some point during said spontaneous infiltration. 14.The method of claim 13, further comprising the step of supplying atleast one of an infiltration enhancer precursor and an infiltrationenhancer to at least one of said matrix metal, said permeable mass andsaid infiltrating atmosphere.
 15. The method of claim 14, wherein saidinfiltration enhancer precursor comprises a material selected from thegroup consisting of magnesium, strontium, calcium and zinc and saidinfiltrating atmosphere comprises an atmosphere selected from the groupconsisting of oxygen and a nitrogen containing atmosphere.
 16. A methodfor forming a metal matrix composite body, comprising:providing apermeable mass comprising at least one precursor to a second material;converting said at least one precursor to at least one second material;at an infiltrating temperature, infiltrating a molten matrix metal intoat least a portion of said permeable mass to form an at least partiallyinfiltrated mass; thereafter heating said at least partially infiltratedmass to a temperature higher than said infiltrating temperature at whichsaid molten matrix metal at least partially infiltrated said permeablemass; maintaining said higher temperature for a time sufficient to formin at least a portion of said at least partially infiltrated mass atleast one reaction product of at least one member selected from thegroup consisting of said second material and said matrix metal; andcooling said at least partially infiltrated mass.
 17. The method ofclaim 16, wherein said precursor to a second material comprises apreceramic polymer.
 18. The method of claim 16, wherein said at leastone second material comprises silicon nitride.
 19. A method of forming ametal matrix composite body, comprising:forming a permeable masscomprising at least one second material selected from the groupconsisting of oxides, nitrides, carbon and carbides; providing a body ofmolten matrix metal; infiltrating said molten matrix metal into saidpermeable mass at an infiltrating temperature to form an at leastpartially infiltrated mass; thereafter heating said at least partiallyinfiltrated mass to a temperature higher than said infiltratingtemperature; maintaining said higher temperature for a time sufficientto cause a reaction involving said at least one second material, therebyforming at least one reaction product in at least a portion of said atleast partially infiltrated mass; and cooling said at least partiallyinfiltrated mass, at least a portion of which has been modified by saidreaction product.
 20. The method of claim 19, wherein said matrix metalcomprises aluminum and said at least one second material comprisessilicon nitride.
 21. The method of claim 20, wherein said at least onesecond material further comprises at least one material selected fromthe group consisting of carbon and boron carbide, and further whereinsaid at least one reaction product comprises at least two reactionproducts comprising aluminum nitride and silicon carbide.